Kurdish-led forces seized the Isil's main hub of Hajin Friday, a milestone in a massive and costly US-backed operation to eradicate the jihadists from eastern Syria.

The Syrian Democratic Forces secured Hajin, the largest settlement in what is the last pocket of territory controlled by Isil, the Syrian Observatory for Human Rights said.

"After a week of heavy fighting and air strikes, the SDF were able to kick IS out of Hajin," Rami Abdel Rahman, the head of the Britain-based monitoring organisation, said.

The operation was completed at dawn, he said, a day after SDF forces fanned out across the large village in the Euphrates valley.

On Thursday, the last Isil fighters were confined to a network of tunnels and the edges of Hajin, which lies in the eastern province of Deir Ezzor, about 30 kilometres (18 miles) from the border with Iraq.