BEIRUT (Reuters) - An Iraqi Shi'ite militia fighting alongside Syrian government forces against Islamic State advanced into Syria's eastern Deir al-Zor province on Friday, the Syrian Observatory for Human Rights monitoring group reported.
The advance led by Harakat al-Nujaba drove Islamic State fighters further back into their stronghold province of Deir al-Zor. Many IS fighters have moved there from Raqqa, where the group is under pressure separately from U.S.-backed militias, according to monitors and people displaced by fighting.
It expanded the territory controlled by Damascus allies in an area close to the Iraqi and Jordanian borders and also around a garrison used by U.S. forces and militiamen supported by Washington.
Tension has risen in the area in recent weeks between Syria, its allies Moscow and Iran, and the United States.
The U.S. military has repeatedly warned forces fighting on President Bashar al-Assad's side to keep away from the garrison, and warplanes from the U.S.-led coalition fighting Islamic State have struck pro-Syrian government forces.
Russia recently warned the United States it would track as targets coalition aircraft flying west of the Euphrates river, another sign of the tension.
Islamic State is being fought by numerous warring sides in Syria, including U.S.-backed Syrian Arab and Kurdish groups, and the Syrian army and its allies backed by Russia and Iran.
The warring sides are competing to seize territory vacated by Islamic State.
The U.S.-backed fighters along the Iraqi border are separate from the Syrian Democratic Forces (SDF) - those fighting around Raqqa.
Reporting by John Davison; Editing by Gareth Jones