The fate of the cease-fire agreement negotiated by the U.S. and Turkey in northeastern Syria was uncertain Friday, as skirmishes erupted between Turkish and Syrian Kurdish forces and questions arose about the boundaries of a buffer zone from which the Kurds are expected to withdraw.

President Trump said his Turkish counterpart, Recep Tayyip Erdogan, told him in a phone call that “minor sniper and mortar fire” had stopped. Mr. Erdogan dismissed the clash reports as “disinformation.” A U.S. official said most of the fighting...