BEIRUT:  Around 150 Daesh fighters have surrendered to US-backed forces in Baghouz, the group's last enclave in eastern Syria, the Syrian Observatory for Human Rights war monitoring group said on Monday.

The Daesh fighters were among 400 people to leave the area on Monday after the US-backed Syrian Democratic Forces staged an assault in recent days, the Observatory said.

US-backed Syrian fighters said they were forced to slow their advance because the extremists were using civilians as human shields.

But despite this hindrance, spokesman for the Kurdish-led Syrian Democratic Forces Mustafa Bali tweeted that the battle to retake Baghouz, the last territory in Syria held by Daesh, was "going to be over soon."

An SDF spokeswoman said the offensive had slowed down but pressure is being kept up away from the corridor to prevent fighters from infiltrating or sabotaging the area. She spoke on condition of anonymity because she was not authorized to speak to the press.

The US-backed forces resumed their offensive on Baghouz last Friday, after a two-week pause to allow for the evacuation of civilians.

Retaking the sliver of land would be a milestone in the devastating four-year campaign to end Daesh's self-proclaimed "caliphate" that once straddled a vast territory across both Syria and Iraq.

"We're slowing down the offensive" due to a small number of civilians held as human shields, Bali said. The previous night, an SDF statement said the Kurdish-led forces would continue their military campaign "to take control of the last ISIS-held pocket in Baghouz and to liberate the remaining civilians who are being used as human shields," using an alternative acronym for the extremist group.

"In order not to harm them, we are advancing slowly but we assert that the battle of Baghouz will end in a short period of time," it said.