Men inspect damage at a site hit by airstrikes in the rebel-held Douma neighborhood of Damascus, Syria, July 27, 2016. Reuters/Bassam Khabieh

The United States on Monday criticised clashes between Turkish forces and some opposition groups in northern Syria as "unacceptable" and called on all armed actors in the fighting to stand down and focus on the fight against Islamic State group.

"We want to make clear that we find these clashes - in areas where ISIL is not located - unacceptable and a source of deep concern," Brett McGurk, the special presidential envoy for the coalition to counter Islamic State, said on his official Twitter account, citing a defense department statement.

"We call on all armed actors to stand down... the U.S. is actively engaged to facilitate such deconfliction and unity of focus on ISIL, which remains a lethal and common threat."

Turkey and its Syrian rebel allies seized territory controlled by Kurdish-aligned forces on Sunday, the fifth day of a cross-border campaign that a monitoring group said had killed at least 35 villagers.

Turkish officials say their goal in Syria is to drive out Islamic State but also to ensure Kurdish militia fighters do not expand the territory they already control along Turkey's border.

The Kurdish YPG fighters are backed by the United States in the fight against Islamic State in Syria. However, NATO member Turkey sees the YPG as an extension of the outlawed Kurdistan Workers Party (PKK), which has waged a three-decade insurgency in Turkey's largely Kurdish southeast.