BEIRUT (Reuters) - Fourteen fighters from the U.S.-backed Syrian Democratic Forces were killed east of the Euphrates river in Syria in a battle with Islamic state militants, an SDF spokesman said on Saturday.

“Daesh terrorists continue to launch counter attacks, taking advantage of the bad weather conditions.... Due to fierce clashes 14 of our fighters were martyred,” Kino Gibrail, the spokesman said in a statement. Daesh is an Arabic acronym for Islamic State.

Several other SDF fighters were wounded in the clashes which began on Friday and were still ongoing on Saturday evening, the statement added. It said U.S-led coalition forces had supported the SDF in fending off the Islamic State attack, and heavy losses were inflicted on the Islamists.

The SDF, which is led by Kurdish militia, is the main ally on the ground of the United States and its coalition partners. Over the past two years, the SDF has helped drive Islamic State fighters from nearly all cities and towns the militants controlled in their self-declared caliphate in Syria.

The SDF now controls a swathe of territory in the north and east of Syria, the largest part of the country outside government control, while largely avoiding direct confrontation with Damascus in the multi-sided war.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, reported that 68 SDF fighters were killed in the clash with Islamic State, including a high ranking officer. More than a hundred others were injured.

Around 2,000 U.S. special forces troops are believed to be on the ground in Syria, where they have aided the SDF.