Militants defending their last bastion in eastern Syria used the cover of bad weather to launch a deadly counterattack against a Kurdish-led force, a war monitor said on Tuesday.

The militant Islamic State group was unable to hold on to the positions they attacked but the assault killed 23 members of the US-backed Syrian Democratic Forces (SDF) and also left nine militants dead.

IS fighters took advantage of poor visibility to unleash suicide attackers on SDF forces along the front line in the Euphrates valley late on Sunday, the Syrian Observatory for Human Rights said.

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“Twenty-three SDF fighters were killed and nine IS militants were also killed in fighting that lasted all night and into Monday morning,” Observatory chief Rami Abdel Rahman said.

The militants often launch attacks under the cover of bad weather that cancels out their opponents' advantage of US-led coalition air power.

The SDF launched what is meant to be the final offensive on the militant organisation four months ago with air and ground support from coalition forces.