The US has said it is "deeply concerned" about a series of Turkish air strikes on Kurdish fighters in Iraq and Syria early Tuesday morning.
More than two dozen fighters died in the attacks, which appear to have hit Kurdish units fighting Daesh who are backed by the US.
American special forces are also embedded with and training Kurdish-led anti-IS forces in northern Syria, leadign to concern about US casualties from unilateral Turkish air strikes in the area.
"We are very concerned, deeply concerned that Turkey conducted air strikes earlier today in northern Syria as well as northern Iraq without proper coordination either with the United States or the broader global coalition to defeat Daesh," Mark Toner, state department spokesman said.
"We have expressed those concerns to the government of Turkey directly."
The Kurdish People's Protection Units (YPG) issued a statement condemning the Turkish attacks.
The group said that Turkish war planes bombed the headquarter's of the YPG's General Command in northern Syria early Tuesday morning.
Bombing destroyed the YPG's communication, media and military infrastructure in Mount Karachok, close to Syria's border with Turkey.
"This trecherous attack has led to the death and wounding of a number of our comrades," the statement read.
"We as the (YPG) say this this cowardly attack will not discourage our determination and our free will to fight and confront terrorism. We also call on our people with all its components in Rojova to stand in line."
The YPG said at least 20 fighters were killed in the 2am strikes, while another bombing also hit Kurdish Peshmerga fighters in Iraq's Sinjar province.
Turkey claimed 70 "militants" were killed in the attack, and accuses the YPG of being linked to the banned Turkish-Kurdish group the Kurdistan Workers' Party (PKK) which has been fighting a brutal war with Ankara.
The YPG is part of the US-backed Syrian Democratic Forces, a coalition of Arab and Kurdish militias which is battling Daesh in Syria as part of a wider assault on the jihadi group's self-declared "capital" Raqqa.
Continued ground and air strikes by Turkey and Ankara-backed rebel forces has angered Washington and led to calls for a united front against Daesh.
"We are also cognizant of the threat that the PKK poses to Turkey... But these kinds of actions frankly harm the coalition's efforts to go after Daesh," added the US state department spokesperson.
Baghdad has also condemned Turkish airstrikes on its soil.
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