Syria Democratic Forces (SDF) fighters walk on the rubble of damaged shops and buildings in the city of Manbij, in Aleppo Governorate, Syria, Aug. 10, 2016. Photo: Reuters / Rodi Said / File.

Around 40,000 civilians have left the last shred of territory controlled by Islamic State in Syria, an official with the US-backed force trying to defeat the jihadists said on Wednesday, surpassing initial estimates and delaying a final assault.

The figure comprises people displaced from Hajin, a town on the Euphrates River that the US-backed Syrian Democratic Forces (SDF) captured in December, and adjacent Baghouz, which they have completely surrounded, said spokesman Mustafa Bali.

The SDF is waiting to complete the evacuation before storming Baghouz or forcing a surrender, but there was no sign on Wednesday of it ending imminently. Dozens of trucks ferried more civilians out along dirt track roads on Tuesday.

Since the SDF announced its final assault on Baghouz on Feb. 9, about 15,000 people have come out of the area, a cluster of hamlets surrounded by farmland near the Iraqi border, Bali said.

Most of the remaining Islamic State fighters holed up in Baghouz are foreigners who have dug defensive tunnels, according to the SDF.

Some people coming from Baghouz in recent days have expressed more open loyalty to Islamic State than those who left earlier on, hinting at the risk the group still poses despite its territorial defeat in Syria and Iraq.

The evacuees from Baghouz were being taken to a camp for internally displaced people at al-Hol, a town near the Iraqi border. The SDF wants foreign governments to help repatriate Islamic State militants, saying the burden and risk of holding them is growing.