BAGHDAD (AFP) - Prime Minister Haider al-Abadi on Saturday (Dec 9) declared victory in a three-year war by Iraqi forces to expel the Islamic State in Iraq and Syria (ISIS) militant group that at its height endangered Iraq's very existence.

"Our forces are in complete control of the Iraqi-Syrian border and I therefore announce the end of the war against Daesh (ISIS)," Abadi told a conference in Baghdad.

"Our enemy wanted to kill our civilisation, but we have won through our unity and our determination. We have triumphed in little time," he said, hailing Iraq's "heroic armed forces".

ISIS seized vast areas north and west of Baghdad in a lightning offensive in 2014.

With Iraq's army and police retreating in disarray at the time, Ayatollah Ali Sistani, spiritual leader of the country's majority Shi'ites, called for a general mobilisation, leading to the formation of Hashed al-Shaabi paramilitary units.

Iraq's fightback was also launched with the backing of an air campaign waged by a US-led coalition, recapturing town after town from the clutches of the militants in fierce urban warfare.

"Congratulations to the government of Iraq and the Iraqi security forces on the liberation of all Daesh-held populated areas in Iraq," the coalition said in a statement on Twitter, using an Arabic acronym for ISIS.

However, Hisham al-Hashemi, an expert on militant groups, warned that ISIS still posed a threat by retaining arms caches in uninhabited desert zones.

Iraq's close ally Iran already declared victory over ISIS last month, as the militants clung to just a few remaining scraps of territory.

But Abadi said at the time that he would not follow suit until the desert on the border with Syria had been cleared.

The militants' defeat is a massive turnaround for an organisation that in 2014 ruled over seven million people in a territory as large as Italy encompassing large parts of Syria and nearly a third of Iraq.

On the Syrian side of the border, ISIS is under massive pressure too.

On Thursday, Russia's defence ministry said its mission in support of the Syrian regime to oust ISIS militants had been "accomplished" and the country was "completely liberated".

In the border region, pro-government forces and US-backed Kurdish-led forces are conducting operations to clear ISIS fighters from the countryside north of the Euphrates valley after ousting them from all Syrian towns.

ISIS RETAINS CAPACITY

The head of Iraq's Joint Operations Command set up to fight ISIS, General Abdel Amir Yarallah, gave an update on Saturday to announce that the desert valley of Al-Jazira was under the control of Iraqi troops and the Hashed all the way from Nineveh province in the north to Anbar in the west.

Federal forces "now control the border with Syria from Al-Walid border crossing to that of Rabia", covering a distance of 435km, he said.

Despite the victory announcements, experts have warned that ISIS retains the capacity as an insurgency group to carry out high-casualty bomb attacks through sleeper cells.

It also retains natural hideouts in the deep gorges of Wadi Hauran, Iraq's longest valley stretching from the Saudi border up to the Euphrates River and the frontiers with Syria and Jordan.

The fightback in Iraq kicked off with the "liberation" of Tikrit, north of Baghdad, that had been under ISIS control for nearly 10 months.

The operations have involved both Teheran, through Iranian-trained Shiite militias in the Hashed al-Shaabi coalition, and Washington as head of the anti-militant coalition.

The western cities of Ramadi and Fallujah followed in 2016 before the turning point of the recapture of Iraq's second city of Mosul in July this year after a nine-month offensive led by a 30,000-strong federal force.

Abadi said the battle for Mosul that left the city in ruins and thousands of its residents displaced marked the end of the militants' "caliphate".

Victory was declared at the end of August in Tal Afar, the last major ISIS urban stronghold in northern Iraq, before a final military operation launched last month against ISIS in a vast desert region of western Iraq.