KULAJO, IRAQ (WASHINGTON POST) - In caves tucked into craggy cliffs and tunnels dug deep beneath the desert, the remnants of a vanquished army are converging for what they hope will be the next chapter in their battle for an Islamic State.

Hundreds and perhaps thousands of Islamic State of Iraq and Syria (ISIS) fighters have made their way over recent months into a stretch of sparsely populated territory spanning the disputed border between the Kurdistan region and the rest of Iraq, according to US and Kurdish officials.

Off limits to Kurdish and Iraqi security forces because of historic disputes over who should control it, this area of twisting river valleys dense with vegetation has attracted the biggest known concentration of ISIS fighters since they lost control of the last village of their once-vast caliphate in eastern Syria in March.

In recent weeks, they have been stepping up their attacks, focused on an area of north-eastern Iraq in the province of Diyala near the border with Iran, carrying out ambushes by night and firing mortars.

Grasses taller than men provide cover for snipers who sneak up on checkpoints and outposts. Government neglect and long-standing grievances foster a measure of sympathy among local residents.

"They have good military plans, they attack when you don't expect them, and they are posing a real threat to people's lives," said Major Aram Darwani, the commander of Kurdish peshmerga military forces in the area.

Across many parts of the vast territory it once controlled, ISIS is scrambling to reassert its presence in a setting that is no longer as welcoming as it once was. Militant fighters who escaped from the battlefield are assembling in ungoverned spaces such as the no man's land between areas controlled by Kurdish and Iraqi forces. Others are laying low as so-called sleeping cells in cities such as Raqqa in Syria, waiting for the phone call ordering them to attack.

Recent visits to the ISIS' former capital of Raqqa and the viciously contested frontier town of Kulajo revealed the challenges the militants face as well as the reemerging threat they pose.

So far, this is less a resurgence than a struggle to survive in the wake of the massive defeat inflicted on the last vestige of their territorial caliphate, according to US military officials.

ISIS remains a long way from possessing the capacity to retake territory, said Brigadier General William Seely, who commands US-led coalition forces in Iraq.

"These are people who are hiding out. They only come out at night to harass and take pot shots," he said. "You can't run a revolution or create your own caliphate if that's all you do."

Over the past two years, tens of thousands of ISIS fighters have been killed, their leadership has been decimated and their self-proclaimed "caliph," Abu Bakr al-Baghdadi, is dead, blown up after he detonated a suicide belt during a US raid on his hideout in October.

As many as 30,000 suspected ISIS fighters are in prison in Iraq and Syria and tens of thousands of their wives and children are detained in dismal camps, according to Kurdish, Iraqi and UN officials.

The group has struggled to reassert itself in its former city strongholds such as Raqqa and Mosul in Iraq, where ISIS attacks have become rare. Memories of its brutal rule and the horrors of the airstrikes used to dislodge the militants deter any desire to see them return, according to Mr Rasha al-Aqeedi, the editor of Irfaa Sawtak, an Iraqi newsletter.

Since US-led forces began to roll back the caliphate more than four years ago, the number of attacks carried out by ISIS in Iraq and Syria has declined, by between 30 per cent and 40 per cent a year since 2016 in Iraq, according to the US-led coalition.

But the militants have already proved adept at infiltrating ungoverned spaces, such as the gap between Kurdish and Iraqi army lines, said Major Johnny Walker, spokesman for the US Special Operations forces that conduct most of the anti-ISIS operations.

"While Daesh is at a serious disadvantage, finding it while it's hiding in the complex human and physical terrain is a complex task requiring significant resources," he said, using the Arab acronym for the ISIS.

ISIS also appears to be gaining momentum in Syria's eastern Deir al-Zour province, where the group made its last stand in March and where tribal and ethnic rivalries help sustain support for the militants.

Assassinations have been on the rise in recent weeks, in part because the US-allied Syrian Democratic Forces pulled fighters out of the area to confront Turkish troops to the north, according to an employee of a US- backed NGO in the province, who was interviewed during a recent trip to the area and spoke on the condition of anonymity due to safety concerns.

Over a typical Syrian breakfast in one of the towns ISIS once ruled, he described having to take back roads through the desert to avoid a cluster of towns where the militants still command loyalties. The group is now making a strenuous effort to rearm, he said.

ISIS fighters have also found refuge in the vast, barely populated desert known as Badia that lies across the Euphrates River from where US troops are deployed. The area is nominally under Syrian government control, and there are indications that the militants there have established a measure of command over cells elsewhere in the country, Syrian Kurdish officials say.

For now, fewer people are being killed in ISIS attacks than in the anti-government protests in Iraq and the battles unleashed by Turkey's invasion of north-eastern Syria in October.

But these new conflicts illustrate the danger posed by the group's residual presence, analysts and military officials say. ISIS owed its conquest of territory to the collapse of state authority over a big part of Syria and the implosion of the Iraqi army in Iraq. Any further deterioration of security in Iraq or Syria would create a new opportunity for ISIS fighters hiding out or laying low.

The militants have not gone away and could yet rise again, cautioned Major General Eric Hill, who commands US Special Forces in Iraq and Syria.

They are making every effort to do so. Over the eight months that Muawiyah Abdul Khader Akraa operated as part of a secret ISIS cell in Raqqa, he said he participated in 17 attacks. He doesn't know how many people he killed because, he said, he didn't linger to find out whether his victims died.

"I did it to avenge our brothers in the battles," he said, displaying no remorse during an interview at the prison in the town of Tabqa, where he has been detained by Kurdish security forces since his arrest in August.

He and two other self-confessed members of the cell agreed to be interviewed in the presence of Kurdish officials, who said they had verified the information the prisoners had provided after months of interrogations. Their accounts offer a rare glimpse into the world of the ISIS' sleeper cells, which lie at the heart of its efforts to reassert its influence in the cities from which it's been driven out.

Akraa, 22, said his missions were assigned at meetings arranged during hurried calls over the encrypted Telegram app. He would be told a time and place to rendezvous, typically a landmark such as the clock tower, a park or Naim Square, where the ISIS carried out public beheadings during its rule over Raqqa.

There he would be met by an "emir" - a prince or leader - who picked him up in a car and would deliver the orders, usually to plant a bomb but sometimes to assassinate a local official.

Akraa said he had been fighting with ISIS in Deir al-Zour province when he was approached by an emir in the area and asked to become an undercover operative in Raqqa. Akraa was given a fake ID identifying him as a Raqqa resident and assigned a smuggler to escort him across the front lines.

After arriving in Raqqa in January, Akraa was introduced to the head of the cell, whom he knew only as Baraa. He gave Akraa 25,000 Syrian pounds (S$65.78) to rent an apartment, the promise of a US$200 (S$271) a month salary and a small bomb, which he was instructed to plant outside a bakery whose owner had refused to pay "zakat," or tax, to ISIS.

The bomb exploded at night and caused no casualties. "It was only a warning," Akraa said. "He paid the zakat."

Working with two others, he embarked on a series of attacks, he recounted. On one day, it was to detonate a bomb in a vegetable cart near a hospital. On another, the task was to drive up to the home of a local official on a motorcycle, knock on his door and shoot him when he came to answer it.

In May, Akraa participated in the biggest attack of the year in Raqqa, setting off a small bomb in Naim Square to attract security forces, which were then targeted in a larger suicide bombing. At least 10 people were killed.

The two other prisoners interviewed said they had been recruited in June, several months after sneaking away from the ISIS' last battle.

Ibrahim Hassan al-Haji said he received a Telegram message out of the blue telling him to report to an emir in a Raqqa park, who informed him he was being activated to be part of a secret cell and offered him a salary of US$80 a month.

He said he complied because he had been unable to find a job and had no money "and because my ideology is jihad."

The third man said he was recruited after he sought the help of an ISIS smuggler to free a relative from the al-Hol camp, where tens of thousands of people related to former Islamic State members are detained. He said he had no choice but to follow the group's orders. "They knew where I lived," he said.

The emirs changed frequently. In April, Baraa disappeared, and a new leader known as "the doctor" showed up to arrange the bombing of Naim Square, said Akraa. Then "the doctor" vanished and was followed by two more.

Then Kurdish forces infiltrated the cell, and one day in August, they burst into Akraa's apartment and detained him. The two others were apprehended shortly after, as were eight other members of the cell.

Attacks in Raqqa have fallen off since the cell was cracked. There hasn't been an assassination inside the city since June, according to Mr Raizin Dirki of the Raqqa Internal Security Forces, which is affiliated to the Syrian Democratic Forces. The only significant bombing came in early October, when three ISIS suicide bombers tried to storm a Kurdish intelligence office where Islamic State prisoners were detained.

None of the cell's emirs have been tracked down, however, said Mr Heval Sharwan, the commander of the unit responsible for rounding up the cell. The captives have told him that two emirs relocated to Turkish-controlled territory in the Syrian province of Aleppo, while others are thought to be hiding out in the Badia desert, where ISIS is believed to coordinate its sleeper cells throughout northeastern Syria.

"We haven't arrested any of the brains," said Mr Sharwan, referring to the leaders. "So we cannot confirm that Raqqa is safe."

Kulajo, a tiny, drab town of flat-roofed concrete homes, lies along one of Iraq's most fraught fault lines in the troubled province of Diyala.

Arabs and Kurds have wrangled over territory here since the former Iraqi president Saddam Hussein began settling Arabs in the area in the 1980s, as part of his campaign to quell the rebellious Kurds. And the area has long been home to Islamic insurgents, including al-Qaeda, which preceded ISIS, according to Mr Darwani, the peshmerga commander, who has been fighting the militants in the area for the past 12 years.

Today, Kulajo is populated mostly by Arabs but is under the control of Kurdish peshmerga. The Iraqi army mans a checkpoint about a mile farther south. But in some spots along the disputed Iraq-Kurdistan border, the no man's land between the two forces is as wide as 20 miles. It is in that space that ISIS fighters lurk, Mr Darwani said.

Earlier this month, he escorted a Washington Post reporter to the town, in his family's pickup truck, because he said a military vehicle would attract unwelcome attention.

Three nights before, three of his men had been killed in an ambush.

Pausing the pickup at the spot where they died, Mr Darwani described the terrifying event. A dense fog had reduced visibility and diminished the ability of the US-led coalition to launch airstrikes in support of his troops. ISIS fighters hiding in the palm groves barely 200 yards away had first fired mortars into the town. When peshmerga reinforcements arrived, they were gunned down.

At a peshmerga post on the edge of the town, little more than a ring of sandbags atop an earth mound, Kurdish fighters said they felt vulnerable, armed only with the Kalashnikov rifles common across the country.

ISIS fighters, however, have mortars and sniper rifles with infrared sights enabling them to strike at night, said Mr Burhan Nouri Hamasayi, one of the post's guards, pointing to the palm groves nearby. "They could easily kill us all," he said.

Mr Darwani put the number of ISIS fighters in his area at about 300 but said he believed many more people in the area were sympathetic to them. "These were the Arabs supported by Saddam when he was oppressing Kurds. They will join any group that is against us. Even people who say they are with us are secretly with Daesh," he said.

As many as 3,000 fighters have gathered along the 150-mile length of the no man's land running between the Iraqi army and the Kurdish peshmerga, according to General Sirwan Barzani, who commands Kurdish forces farther north, in the Qara Chokh mountains. US military officials say they put the number at closer to 500, strung out in remote terrain and operating in groups of around five.

Gen Barzani said the militants are living off the land, shaking down the local villagers for food and money. A local television station, Rudaw, has filmed ISIS fighters clambering down a cliff face in his sector of the no man's land, stripping naked and bathing in a river.

"I don't think the strategy of ISIS now is to do big things. They need more time," he said. "They are reorganising themselves, getting weapons and arms. They don't have the power now to do a big attack."

But the difficult terrain and rivalry between the Iraqi army and Kurdish forces preclude any kind of organised offensive to root out the ISIS militants, said Mr Darwani.

"Iraq is on the edge of a cliff, and it is falling," he said, urging a hasty departure from Kulajo as the sun set. For the ISIS to return, he added, "it is a matter of time."