Syrian government troops and allied militia have cleared the last pockets held by the Islamic State group on the west bank of the Euphrates River, a monitor said on Thursday.

But the jihadists still hold areas on the east bank in the north of oil-rich Deir Ezzor province, the Syrian Observatory for Human Rights said.

"Daesh no longer has a presence on the western side of the river," Observatory head Rami Abdel Rahman told AFP, using an Arabic acronym for IS.

"Its biggest presence is on the eastern side of the Euphrates, where it still holds around eight percent of Deir Ezzor province," he said.

State news agency SANA said late Wednesday that Syrian troops and allied forces had captured around two dozen towns and villages on the west side of the river.

East of the Euphrates, IS is under attack by both government troops and Kurdish-led militia.

IS also has a presence in eastern parts of the central provinces of Homs and Hama, and in the Yarmuk Palestinian camp near Damascus, while an allied group holds positions in the south.