Thousands of civilians, mostly relatives of fighters, are fleeing the militant Islamic State (IS) group's last stronghold in eastern Syria, a war monitor said on Thursday.

According to the Syrian Observatory for Human Rights, 11,500 people have fled the area since Kurdish-led forces broke IS defences and took the militants' main hub of Hajin two weeks ago.

“The past fortnight saw the biggest exodus” since the launch in September of a broad offensive against the IS by the Syrian Democratic Forces (SDF), the monitoring group said.

The outfit is an alliance of the Kurdish militia which controls northeastern Syria and local Arab fighters that operates with backing from a US-led military coalition.

The militant 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 militants' last redoubt, known as the Hajin pocket, on Sept 10. They took the town of Hajin on Dec 14, after months of an offensive slowed by Turkish threats against the Kurds further north as well as fierce counter-attacks by IS fighters with little to lose.

“Most of the displaced are IS relatives,” said Rami Abdel Rahman, the head of the Britain-based Observatory.

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

He said that since the loss of Hajin, the last town of note in the area, the militants have been unable to defend their positions and were quickly falling back.

IS still controls the villages of Al-Shaafa and 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.

While it could soon lose its last fixed positions, the IS remains a threat, with roving units still carrying out attacks from their desert hideouts and cells reportedly regrouping in several parts of Iraq and Syria.