The Kurdish People's Protection Units (YPG) have reinforced Arab SDF forces in an offensive against IS in eastern Syria after the jihadists retook territory there.

IS launched a counter-offensive near the Syria-Iraq border on Friday in Hajin, located in Deir az-Zour province.

Around 70 SDF fighters died since the jihadists launched their counter offensive on Friday, the Syrian Observatory for Human Rights monitoring group said.

The US-led coalition fighting against IS said it would "come back with coalition" support, according to a spokesman.

An SDF commander said the weekend setback was a result of inexperienced Arab SDF forces, which are playing a large role fighting IS in Deir az-Zour.

While the US-backed SDF has been closing in on IS' last pockets in eastern Syria, the YPG has said it was forced to deploy more forces against the battle-hardened IS there.

"They will be relied on to complete the campaign," an SDF commander said, according to Reuters.

The US-led coalition is seeking to clear IS from its last remote outposts east of the Euphrates River and along the Syria-Iraq border.

"This battle is give and take sometimes like most military fights and we have been saying from the beginning, this will be a difficult struggle," said Colonel Sean Ryan, the coalition's spokesman.

The Shiite Iraq's Popular Mobilisation Forces said over the weekend it would reinforce its side of the Syrian border.

The SDF, who are backed by airstrikes of the US-led coalition, launched its campaign to retake the IS holdout of Hajin in Deir az-Zour on 10 September.

But they have faced a fierce fightback from the militants, including under the cover of sandstorms.

More than 300 SDF fighters and around 500 IS jihadists have been killed in the past seven weeks of fighting, the Observatory says.

The coalition estimates that 2,000 IS fighters remain in the Hajin area.