President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE urged Turkish President Recep Tayyip Erdogan to scale back military operations in a Kurdish enclave in northern Syria during a phone call Wednesday.

Trump cautioned Erdogan against taking any actions that could risk a potential clash between Turkish and American forces, according to the White House.

Trump also said Ankara and Washington should instead keep their focus on defeating ISIS militants.

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The call came four days after Turkish forces launched an attack into Afrin, a city in northwestern Syria which is controlled by a Kurdish group known as the YPG.

The Turkish government regards the YPG as an insurgent group and a threat against Turkey.

YPG fighters, though, also comprise most of the U.S.-backed Syrian Democratic Forces, which worked with the U.S.-led international coalition to clear ISIS militants from much of northeastern Syria.

The U.S. does not back Syrian Kurdish fighters in Afrin, but the Turkish attack in the region and Erdogan's pledge to expand operations have raised tensions between Washington and Ankara.