Syria war: 'Final assault' launched to recapture Raqqa

US-backed militias have been besieging the city for months

US-backed forces have launched their final assault on the remaining Islamic State (IS) militants in Raqqa.

The Syrian Democratic Forces (SDF) says it began the offensive on Sunday, after a convoy of IS fighters and their families left the city, along with hundreds of civilians.

No foreign fighters were allowed to join them, and now make up most of the remaining 200 to 300 militants, SDF spokesman Talal Selo said.

Raqqa was IS's de facto capital.

It was one of the first large cities the group took over in 2014, and had held control there for three years.

But the SDF, an alliance of Kurdish and Arab militias, has been besieging the city for nearly four months.

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In a statement released as the battle began, the group said they would not stop "until the whole city is clean of the terrorists who refused to surrender, including the foreign fighters".

The battle is still anticipated to take some time, with Colonel Ryan Dillon, spokesman for the US-led coalition, saying they "still expect there to be difficult fighting".

The SDF's decision to allow some of the militants to leave the city, leaving only a hardcore group of fighters behind, was designed to shorten the fight.

The coalition said "275 local mercenaries and their families" had left Raqqa. Omar Alloush, an official in the Raqqa Civil Council, said about 400 civilians also joined them.

Mostafa Bali, an SDF spokesman, told Reuters they were human shields who the militants refused to release until their own safety was guaranteed.

Mr Selo said the convoy was still in SDF territory on Sunday afternoon.

But the evacuation did not have the full support of their western backers.

Col Dillon said: "We may not always fully agree with our partners at times. But we have to respect their solutions."

The loss of Raqqa will be seen as another blow for IS, which has been steadily losing ground in both Syria and Iraq over the last two years.

IS, which attracted fighters from across the globe with its extreme interpretation of Islamic law, used beheadings, crucifixions and torture to terrorise residents who opposed its rule.