Baghouz (Syria), Feb 21 (AFP) A deadly car bombing claimed by the Islamic State group hit US-backed forces in east Syria Thursday as they tried to negotiate the release of civilians trapped in the jihadists' last sliver of territory.

The Syrian Democratic Forces (SDF) are working towards evacuating civilians remaining in the holdout, so they can polish off the dying IS "caliphate" whether through an assault or a surrender deal.

The jihadists overran large parts of Syria and neighbouring Iraq in 2014, but several offensives have retaken all but half a square kilometre (a fifth of a square mile) of the territory in the eastern Syrian village of Baghouz.

A spokesman for the US-led coalition fighting IS said international forces "continue to support the SDF as they negotiate having innocent civilians released" and their captured fighters returned.

As the SDF pressed the last IS diehards a car bomb killed 14 oil workers and six of the Kurdish-led alliance's conscripts near the Omar oil field which it uses as its main base in the region, the US-backed group and a monitor said.

IS claimed responsibility for the blast on Telegram, saying its fighters had planted and detonated the explosive-laden car.

SDF spokesman Adnan Afrin said the explosion in the village of Shheel, some 100 kilometres (60 miles) north of Baghouz, was another example of IS cells attacking its fighters behind the front line.

"This is what we've been talking about in the past... about sleeper cells trying to impede our progress in Baghouz," he said.

A day after hundreds of people were evacuated from the last IS remnant, more than 50 trucks on Thursday returned near empty from Baghouz to SDF territory, an AFP correspondent said.

"We couldn't enter Baghouz," said a man who had accompanied the convoy.

"We got to an SDF point and we found around 15 people -- women and children including a French woman and an Egyptian woman. We took them," he said.

Thousands of people have escaped IS territory in recent weeks, but the flow slowed to a trickle at the weekend, before Wednesday's first batch of evacuees.

Paul Bradley, from the Free Burma Rangers volunteer group, said people fleeing painted a grim picture of life inside.

"They showed us this bread that's basically mashed up wheat with water burnt on both sides, $16 a kilo," he said.

SDF spokesman Afrin said most of those trucked out on Wednesday were civilians, but they also included IS fighters.

On Thursday, the AFP reporter saw hundreds of people waiting in a screening area where the SDF have been questioning new arrivals in recent weeks, to separate out suspected jihadists from the civilians.

The Syrian Observatory for Human Rights, a Britain-based war monitor, said Wednesday that negotiations were being held "for the surrender of the last IS fighters".

It said there were "reports of a deal" but the details were unclear.

At the height of its rule, IS imposed its brutal ideology on a territory roughly the size of the United Kingdom, attracting thousands of supporters from abroad.

But the jihadists have since lost almost all their territory, and hundreds of foreigners suspected of being IS fighters, as well as related women and children, are being held by the SDF.

Other foreign members have been killed. A top French jihadist, who voiced an audio recording claiming responsibility for the November 2015 attacks in Paris, was killed in an overnight airstrike, security sources told AFP on Thursday.

Fabien Clain, who is believed to have gone to Syria in March 2015, was killed in Baghouz, they said.

Across the border, security officials in Iraq said the SDF handed over 130 Iraqi jihadists to Baghdad on Thursday, but SDF spokesman Mustefa Bali denied the claim.

Syria's Kurds have long demanded the repatriation of foreigners accused of belonging to IS in their custody, but their home countries have been reluctant.

US President Donald Trump said Wednesday he was barring a US-born former IS propagandist from returning home from Syria, where the conflict has killed more than 360,000 people and displaced millions since 2011.

Secretary of State Mike Pompeo said Thursday that the diplomatic status of the father of Alabama woman Hoda Muthana means she is not a US citizen.

It came after a lawyer for the family of a teenager who fled London to join the jihadists when she was 15 said Britain was revoking her citizenship.

Shamima Begum, 19, at the weekend gave birth to her third child. Two previous children died at an early age. (AFP)

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