Thousands of civilians, mostly relatives of extremist fighters, are fleeing the "Islamic State of Iraq and Syria" (ISIS) group's last stronghold in eastern Syria, AFP reported Thursday (December 27th).

According to the Syrian Observatory for Human Rights, 11,500 people have fled the area since the Syrian Democratic Forces (SDF) broke ISIS defences and took the extremists' main hub of Hajin two weeks ago.

"The past fortnight saw the biggest exodus" since the launch in September of a broad offensive against ISIS by the SDF, the Observatory said.

The extremist group had already lost all of its major urban centres earlier in 2018 but was clinging to the remote area in the Euphrates River Valley.

The SDF launched an operation involving more than 15,000 fighters to smash the extremists' last redoubt, known as the Hajin pocket, on September 10th.

The Arab-Kurd alliance took Hajin on December 14th.

"Most of the displaced are ISIS relatives," said Observatory head Rami Abdel Rahman.

He added however that fighters were attempting to blend in with civilians to save their lives and that the SDF had managed to detain 700 so far.

Since the loss of Hajin, ISIS elements have been unable to defend their positions and were quickly falling back, he said.

ISIS still controls the villages of al-Shaafa and al-Sousa as well as a handful of hamlets dotting the eastern bank of the Euphrates.

Abdel Rahman said he expected the last rump of what was once a sprawling "caliphate" straddling swathes of Iraq and Syria to collapse in the coming days.