AIN ISSA, Syria/AMMAN (Reuters) - People from Raqqa are fleeing their city under cover of night as U.S.-backed forces close in on the Islamic State stronghold, running a gauntlet of minefields and hostile fighters instead of risking death in a major battle expected to begin soon.

Islamic State has used threats and coercion to stop people leaving, forcibly returning some to the city in an apparent effort to use them as human shields against the looming assault by the U.S.-backed Syrian Democratic Forces.

At a camp north of Raqqa, Syrians who escaped the city in the last few days say Islamic State checkpoints are now disappearing as fighters either leave for the front lines or withdraw to strongholds to the south.

“There are no longer checkpoints. There were before, but they evacuated,” said Abdullah Hamad Ali, who escaped Raqqa on Saturday night by foot, guided by a smuggler to whom he paid $2,100 to get his family out of the city.

“We walked through farmland planted with mines,” he told Reuters at the camp in Ain Issa, a town around 45 km (30 miles) north of Raqqa where more than 3,000 people from the city are sheltering in SDF-held territory.

The assault on Raqqa promises to be a defining moment in the U.S.-led war against Islamic State in Iraq and Syria. Along with the Iraqi army’s campaign to drive Islamic State out of Mosul, it threatens to deal a major blow to the militants.

It is unclear how many people remain in Raqqa, Islamic State’s Syrian base of operations for more than three years and a major symbol of the cross-border “caliphate” in Iraq and Syria. The city’s population has been estimated at 200,000 before the recent departures.

The U.S.-led coalition and SDF are still expecting a tough fight. Islamic State has been digging fortifications and trenches in the city in readiness for an attack which the Kurdish YPG militia has said will begin in April.

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QUESTIONING CHILDREN

As civilians struggle to leave, Islamic State is doing its best to stop them.

The jihadists have executed at least one man for smuggling families out of Raqqa, said Abdullah Shabaan, a former resident who fled earlier this year but remains in contact with people in the city. He said he had given up trying to get relatives out of Raqqa for now because of the risks.

Fighters at remaining Islamic State checkpoints on roads out of the city have been asking children rather than their parents where they are going in an effort to catch them out, said Ali Mohamad, speaking at the Ain Issa camp. People found heading to SDF areas have been sent back into Raqqa and their drivers detained.

Barricades and fortifications have been erected in the city.

“The situation in Raqqa was terrible,” said Mostafa al-Ahmad, who also left Raqqa on Sunday for Ain Issa. “The SDF is the one advancing, so we thought, ‘let’s just go over to them and get it over with’,” he said.

Many in Raqqa are hoping to see the back of Islamic State, which has imposed an ultra-hardline interpretation of Islam and brought foreign militants from all over the world to their city. Many of those foreigners have now gone, according to residents.

But Islamic State has been whipping up fear of the alternative, for months telling people in Raqqa that the YPG was coming to commit atrocities against them, said one resident, speaking via the internet from the predominantly Arab city.

The SDF dismisses this as IS propaganda designed to discourage the people of Raqqa from fleeing to SDF-held areas.

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PRICES SOAR

Islamic State has added to the panic in other ways.

Its warning that the Tabqa Dam, 40 km (25 miles) up river, was in danger of collapse triggered panic in the city. Many rushed to high ground to escape a flood that never came.

The scare led thousands to sleep in the desert hills on the city’s outskirts, according to three former residents of Raqqa who had heard the details.

Islamic State said the U.S.-led coalition had bombed the dam, which is under IS control. Both the SDF and U.S.-led coalition denied that, and say there is no danger of collapse.

The growing intensity of coalition air strikes has been another big worry in the city. The Syrian Observatory for Human Rights has reported a spike in civilian deaths from air strikes in the Raqqa area in March.

In one incident, it said 33 people were killed at a school near Raqqa. The coalition says militants were killed.

Residents say Islamic State military positions including weapons stores have been placed near civilian buildings, including a bakery that was hit in recent weeks.

Prices are meanwhile sky-rocketing due to an ever tightening siege. Bridges across the Euphrates River, which borders Raqqa to the south, have all been destroyed. Produce is brought across the river by boat, and costs four times what it used to.

People in contact with relations in the city say they are asking to be sent money just to survive. “It’s too expensive to buy food these days,” said Raed Bani Amer, who spoke to his family in Raqqa in recent days.

Mazen Hasoun, an activist from Raqqa in Europe who is in touch with friends and relatives in the city, said the militants were busy digging trenches, even in the backstreets.

A well-known Saudi cleric was among the foreign members of the group no longer seen in the city, said one resident, who had heard the news from his father.

“I hear the Sheikh is busy with the Mujahideen on the front lines,” he said. “He has no time to deliver sermons these days.”