The U.S. State Department has expressed "deep concerns" about Turkish air strikes against Kurdish fighters in northern Syria and northern Iraq, saying the strikes were not approved by the U.S.-led coalition fighting Islamic State (IS) extremists.
The State Department said it had communicated its concerns directly to the Turkish government on April 25.
The air strikes have led to calls from Syrian Kurdish members of the U.S.-led coalition to call for U.S. aerial protection against Turkish air strikes.
Iraqi Kurdish authorities also demanded the withdrawal of Kurdistan Workers Party (PKK) fighters from northern Iraq's Sinjar region after the attacks.
Turkey confirmed that its warplanes bombed Kurdish militants in Iraq's Sinjar region and in northeastern Syria.
A Turkish military statement said the strikes in Sinjar were aimed at preventing the PKK from sending weapons and explosives for attacks inside Turkey.
Turkish President Recep Tayyip Erdogan said late on April 25 that he will not allow the Sinjar region to become a base for PKK militants.
But the U.S. State Department said on April 25 that the Sinjar air strikes killed members of the Iraqi Kurdish Peshmerga forces, considered an important ally in the U.S.-led coalition against IS militants.
A senior Kurdish military commander, General Seme Bosali, said five Peshmerga fighters were killed in Sinjar.
While the Peshmerga and Syrian Kurdish fighters from the People's Protection Units (YPG) are part of the anti-IS coalition, the PKK has been designated a terrorist group by Turkey, the United States, and the European Union.
The PKK established a presence in the Sinjar region during the summer of 2014 when its fighters helped tens of thousands of Yazidis escape Mount Sinjar, which was encircled by IS militants at the time.
Turkey has regularly bombed the border area between Iraq and Turkey where PKK militants are based, but the April 25 air strikes marked the first time Turkey has targeted Kurdish fighters in the Sinjar region.
Meanwhile, the air strikes in northeastern Syria targeted Syrian Kurdish fighters from the YPG, a key component of the Syrian Democratic Forces (SDF), which are backed by the United States and have been closing in on the IS stronghold of Raqqa.
The YPG said in a statement that parts of its headquarters in Mount Karachok were hit near Syria's border with Turkey, including a media center, a local radio station, communications facilities, and military institutions.
YPG spokesman Redur Xelil said 20 YPG fighters were killed and 18 were wounded, three of them critically.
Reuters reported that a U.S. military officer accompanied YPG commanders on a tour of the targeted sites later on April 25, a visit that demonstrated the close partnership between the U.S. military and the YPG.
Ilham Ahmed, a senior Kurdish politician who co-chairs the political wing of the SDF, said on April 25 that the SDF had called on Washington provide protection against Turkey.