US-backed militias said they had launched their final assault on Syria’s Raqqa on Sunday after a convoy of ISIS militants left the city, leaving only a hardcore of militants to mount a last stand.

“The battle will continue until the whole city is clean,” said a statement by the Syrian Democratic Forces (SDF), a US-backed alliance of Kurdish and Arab militias.

The SDF said earlier that a group of the militants had left in a convoy taking some civilians with them. But there were conflicting accounts as to whether the evacuees included both Syrian and foreign fighters.

Raqqa’s fall to the SDF now looks imminent after four months of battle.

“We still expect there to be difficult fighting,” said Colonel Ryan Dillon, spokesman for the US-led international coalition backing the SDF in the war against Islamic State.

Raqqa was the first big Syrian city to fall to ISIS as it declared a “caliphate” and rampaged through Syria and Iraq in 2014, becoming an operations centre for attacks abroad and the stage for some of its darkest atrocities.

But ISIS has been in retreat for two years, losing swathes of territory in both countries and forced back into an ever-diminishing foothold along the Euphrates river valley.

“Last night, the final batch of fighters (who had agreed to leave) left the city,” said Mostafa Bali, an SDF spokesman.

Bali said only Syrian ISIS fighters had evacuated in the convoy. But Omar Alloush, an official in the Raqqa Civil Council formed under SDF auspices to oversee the city, said some foreign fighters had also departed.

Neither said how many fighters had left or how many remained in the tiny, bomb-cratered patch of Raqqa still held by ISIS. Before the convoy left, the coalition estimated that about 300-400 fighters remained.