AL-RAI, Syria (REUTERS) - On a road in northern Syria, a rebel fighter signals to a group of men, women and children traipsing across barren fields to put their hands in the air. He pats them down and inspects their phones, trying to determine whether they are Islamic State in Iraq and Syria (ISIS) sympathisers.
The group - two families from the town of Tel Afar near Mosul in Iraq - are part of a rising tide of people flooding into northern Syria, fleeing deteriorating conditions and conflict in the parts of Iraq and Syria still controlled by ISIS as operations to crush the militants gather pace.
"Praise God, we were only three days on the road," said one of those stopped, one of two brothers travelling with their wives and children. "There are people who take them a month and more."
The two families paid US$32,000 (S$43,859) to smugglers who took them to the edge of ISIS territory in north Syria - around 500km - inside oil tankers.
From there, they walked the final 25km to arrive at a Free Syrian Army (FSA) rebel checkpoint just outside the town of al-Rai on the Turkish border, an area of northwestern Syria purged of ISIS by Turkey and its Syrian rebel allies in August.
"We were in the oil tanker for more than nine hours. The women fainted from the heat and lack of oxygen," said the brother, asking not to be named to protect relatives back home. "The children were given medicine to make them sleep so they would not wake when Daesh members tapped the tanks at checkpoints to make sure nobody was inside," he said, using an Arabic term for ISIS.
In a long-anticipated operation aimed at delivering a killer blow to the militants in Iraq, Iraqi forces backed by a US-led coalition could launch an operation to oust them from their stronghold of Mosul as soon as the second half of October.
The anticipated attack has caused a spike in people leaving the surrounding area since May, according to a UN-affiliated body which monitors population movement inside Syria but which asked not to be named to protect its workers.
"They (ISIS) kill us every day," said the brother, who said he aims to cross into Turkey to join relatives. "You have to let your beard grow. If you do anything wrong, they will fine you 50,000 or 100,000 Iraqi dinars (S$117). They fine you constantly."
While Iranian-backed Shi'ite militias allied to Baghdad's Shi'ite-dominated government have been a key deterrent against the hardline Sunni militants, they have also aroused fear as they move into areas dominated by Sunni Muslims, such as Tel Afar.
"The militias would consider us to be Daesh even if I told them that we are civilians," said the brother. "We are powerless to escape the double fear of Daesh and the Iranian and Shi'ite militias."
The rebels manning the checkpoints at al-Rai are also wary about who is and isn't ISIS. Rebel fighter Abu Muhammad's job is to clear refugees for entry to the town.
"When we capture a Daesh member from among the fleeing civilians, we interrogate him and try to find out if he has sleeper cells among us," he said. "In the event that someone renounces Daesh, we treat him well. If he has family we try to secure them, even if they are still in Daesh territory."
Arrivals cleared for entry are given aid and free transport out of al-Rai to other rebel-held towns, he said.
Another refugee, a 35-year-old Syrian who gave his name as Muhammad, said he travelled around 50km from an ISIS-controlled area near Manbij, a city in northern Syria cleared by a US-backed alliance of Arab and Kurdish fighters in August.
He paid smugglers 40,000 Syrian pounds (S$109) for each of the four children and two adults who accompanied him on his journey. He said he had no plan for where he was ultimately heading. "The important thing is just that we left," he said.
Now in its sixth year, Syria's civil war has cut the country into a patchwork of territories held by the government and an often competing array of armed factions, including Kurdish militia fighters and a loose coalition of rebel groups.
Hundreds of thousands of people have been killed and 11 million have been displaced - half the country's pre-war population.
The UN-affiliated body told Reuters it knows of at least 6,000 people who fled Raqqa, ISIS' de facto capital in Syria, for other parts of Syria in June, July and August.
The majority of these travelled to ISIS-controlled areas in northwest Aleppo province, getting themselves as close as possible to the border with FSA-controlled territory, with the aim of travelling on to Turkey if possible.
More than 5,500 have also left Deir Ezzor in eastern Syria, the largely ISIS-controlled province which borders Iraq, the group said. The majority of those headed north to Hasaka province, which is controlled by the Kurdish YPG militia.
Many fleeing ISIS-controlled areas around Mosul in Iraq also head over the border to Hasaka.
Mr Ahmad Khader, 26, from Deir Ezzor, said ISIS confiscated his identification papers. Members of his group had to pay 25,000 Syrian pounds per person for the journey to al-Rai, which included walking 40km by foot.
"It is forbidden to leave territory controlled by Islamic State... most cars on the way were scared to carry us just because we are from Deir Ezzor," he said, using another name for ISIS.
Mr Wael al-Jassim, 22, paid smugglers 60,000 Syrian pounds each for himself, his wife and two children to travel from their home in ISIS territory east of Aleppo.
"I got myself smuggled because there was no work. With or without Daesh, if there is no work, how can I feed my family?" he said.
The rebels manning the checkpoints around al-Rai estimated they see at least 3,000 people passing through each day. "Those fleeing are in a pitiful state," said Muhammad, the rebel fighter. "They travel long distances and pay large sums of money to smugglers."