The head of the Syrian Kurdish YPG militia said on Wednesday that Turkish military deployments near Kurdish-held areas of northwestern Syria amounted to a “declaration of war” which could trigger clashes within days.

Turkey’s Deputy Prime Minister Numan Kurtulmus retorted that his country was not declaring war but that its forces would respond to any hostile move by the YPG, which he described as a small-scale army formed by the United States.

The mounting tensions between two U.S. allies in northwestern Syria risk opening yet another front in the multi-sided conflict, in which outside powers are playing ever greater roles.

They could also distract the YPG from the U.S.-backed campaign it is spearheading to capture Islamic State’s stronghold of Raqqa, some 200 km (125 miles) away.