Beirut (AFP) - Over 600 people were evacuated Saturday from the Islamic State group's remaining holdout in eastern Syria, a monitor said, as US-backed fighters prepare for a final assault on the area.

"More than 600 people, mainly women and children, were evacuated on 25 buses sent" by the Syrian Democratic Forces (SDF), the Britain-based Syrian Observatory for Human Rights said.

Observatory head Rami Abdel Rahman said that several dozen jihadist fighters were among those evacuated to areas held by the Kurdish-Arab alliance.

The SDF, backed by air strikes from a US-led coalition, in September launched an offensive to oust IS from the rump of the once-sprawling "caliphate" it proclaimed in 2014.

Abdel Rahman said some 16,000 people, including 760 IS fighters, have fled the area since the start of December.

But "this is the first time that buses have been provided by the SDF and coalition", he said, suggesting a potential deal had been struck between the warring sides.

The United Nations said Friday that overall some 25,000 people have fled the violence over the last six months as the die-hard jihadists have battled to defend their dwindling bastions.

An estimated 2,000 civilians remain trapped in the area around the town of Hajin, the UN said.

The US-led coalition on Saturday fired over 20 missiles against jihadist positions, the Observatory said.

The monitor said some 300 SDF combattants had deployed near the village of Sousa in preparations for a final assault.

The fight to eliminate the remaining pockets of IS has been shaken by President Donald Trump's shock December announcement that he is withdrawing US troops from Syria.

Secretary of State Mike Pompeo on Saturday sought to reassure Washington's Kurdish allies in the fight against IS, who fear the departure of American troops would allow Turkey to attack them.