Turkish planes bombed Kurdish fighters in Iraq’s Sinjar region and northeast Syria on Tuesday, killing at least 20 in a widening campaign against groups linked to the outlawed Kurdistan Workers Party.

A Turkish military statement said around 70 militants were killed in the operations inside the two neighboring states.

The air strikes in Syria targeted 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 Islamic State bastion of Raqqa.

The Turkish raids showed the challenges facing U.S.-led attempts to defeat Islamic State in Syria and risk increasing tension between NATO allies Washington and Ankara over Kurdish combatants who have been crucial in driving back the jihadists.

In Washington, the State Department said it was deeply concerned by the air strikes, which were not authorized by the U.S.-led coalition fighting Islamic State in Iraq and Syria. Turkey is part of the coalition of more than 60 countries.

“We have expressed those concerns with the government of Turkey directly,” State Department spokesman Mark Toner told reporters on a conference call. “These air strikes were not approved by the coalition and led to the unfortunate loss of life of our partner forces,” he added.

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