BEIRUT (AP) — A U.S.-backed Syrian Kurdish militia says two American volunteers were killed fighting the Islamic State group in northern Syria last week.

The People's Protection Units, also known by their Kurdish initials as the YPG, announced Tuesday that 29-year-old Nicolas Alan Warden of Buffalo, New York, died in battle on July 5 in Raqqa, IS' de-facto capital.

The YPG says 28-year-old Robert Grodt from California, was killed on July 6 in the same battle.

The U.S. military has developed a close relationship with the YPG and its parent coalition, the Syrian Democratic Forces, in the war against IS.

The two men were not members of the U.S. special forces embedded with the Syrian coalition.

U.S.-backed forces have conquered a third of Raqqa since launching the push on the city last month.