AIN ISSA, SYRIA (AFP) - United States-backed forces announced the "final phase" of the battle to retake Syria's Raqqa on Sunday (Oct 15), after a group of foreign Islamic State in Iraq and Syria (ISIS) group fighters left their one-time stronghold under an evacuation deal.

Raqqa was once the de facto Syrian capital of the militants' self-styled "caliphate" in Syria and Iraq, but ISIS now holds just 10 per cent of the city.

On Sunday, the US-backed Syrian Democratic Forces militia that has been battling to oust ISIS from the city since entering it in June said the fight was now entering its endgame.

"We are now in the final phase of the battle for Raqqa," said Jihan Sheikh Ahmed, spokeswoman for the SDF's Raqqa campaign.

The militia said in a statement that the last phase of the fighting would "end the presence of the terrorist mercenaries inside the city".

"The battle... will continue until the entire city is cleared of terrorists who refuse to surrender, including foreign terrorists." The announcement comes after a deal brokered by local officials to evacuate ISIS fighters from the city.

The deal was announced Saturday, with one senior local official telling AFP that both foreign and ISIS fighters would be leaving the city, possibly to remaining militant-held territory in neighbouring Deir Ezzor province.

'Fight or surrender'

On Sunday, the official confirmed that a group of foreign fighters had departed.

"A portion of the foreigners have left," said Omar Alloush, a senior member of the local Raqqa Civil Council.

He could not confirm how many fighters had left, or where they had gone.

"They took civilians as human shields and left," he added.

The exact details of the agreement have been murky, with the US-led coalition reporting Saturday that a convoy would be leaving Raqqa but adding that the agreement "purportedly excludes foreign Daesh (ISIS) terrorists".

On Sunday, coalition spokesman Colonel Ryan Dillon said he could not confirm any details on the departure of ISIS fighters.

But he reiterated the coalition's opposition to the departure of foreign fighters, and said that position had been shared with local officials.

"We're very adamant about not allowing foreign fighters to leave the city," he told AFP.

"Our stance was they either stay and fight or they surrender unconditionally." "The last thing we want is foreign fighters to go free so they can return to their countries of origin and cause more terror and more havoc," he added.

String of losses for ISIS

But Dillon said local officials had not been asked for guarantees.

"This is a local solution," he said.

"While we may not fully agree with our partners sometimes, we have to respect their own solutions to their issues." The deal was agreed amid concern about up to 8,000 civilians still trapped in Raqqa, many of them reportedly being used as human shields by ISIS.

Ahmed said many civilians had been able to flee in recent days.

"There are very few left, and they are coming towards our forces at any chance they get," she told AFP.

The SDF statement also referred to the departure of civilians, saying it was beginning the last phase of the battle after a deal to "evacuate remaining civilians in the city and ensure the surrender of 275 local mercenaries and their families".

The SDF began its campaign to capture Raqqa last November, fighting for months to encircle the city before breaking into it in June.

ISIS captured Raqqa in 2014, and under its rule the city become synonymous with the militant group's worst abuses, and was transformed into a planning centre for attacks abroad.

The loss of Raqqa would be only the latest blow for ISIS, which has suffered a string of setbacks in recent months.

It was driven from its largest Iraqi stronghold Mosul in July and now holds only a sliver of territory in the country.

In Syria, its presence is largely confined to the eastern province of Deir Ezzor, where it is under attack by both the SDF and a Russia-backed Syrian government campaign.