Vehicles that belonged to the Kurdish People’s Protection Units were damaged by Turkish airstrikes in Syria. The U.S.-backed group, which is battling the Islamic State, said 20 of its fighters were killed. (Rodi Said/Reuters)

Turkey stepped up its bombing campaign Tuesday against Kurdish militants outside its borders, killing as many as 20 U.S.-backed fighters in Syria and expanding its strikes in Iraq, an escalation that could complicate efforts to combat the Islamic State. 

The predawn raids drew swift rebukes from the United States and Iraq, which accused Turkey of not properly coordinating the strikes. The government of the Iraqi region of Kurdistan described the strikes as “painful and unacceptable” after five of its peshmerga fighters were killed in an apparent misfire. 

The Turkish military said the raids targeted “terrorist hotbeds” and supply routes used by the Kurdistan Workers’ Party (PKK) to smuggle ammunition, arms and explosive material into Turkey, where it is waging an insurgency. 

But the PKK affiliate hit in the Syria strikes — the People’s Protection Units (YPG) — has played a key role in turning back Islamic State militants and is a major component of U.S.-backed forces preparing for an assault on the Islamic State stronghold of Raqqa. In a statement, the YPG said 20 of its fighters were killed. 

“These airstrikes were not approved by the coalition and led to the unfortunate loss of life of our partner forces in the fight against ISIS that includes members of the Kurdish peshmerga,” State Department spokesman Mark Toner told reporters, using another name for the Islamic State. He said the United States was “deeply concerned” about the strikes.  “We have expressed those concerns to the government of Turkey directly,” he added.

Turkish President Recep Tayyip Erdogan’s government is increasingly alarmed by the growing sway of Syrian Kurdish fighters across Turkey’s southern border and has repeatedly asked the United States to abandon its support for the YPG. The incident underscores the increasing tension between Washington and Ankara over the role of the Kurdish fighters, which is complicating planning for the Raqqa offensive.

A defense official, speaking on the condition of anonymity because he was not authorized to discuss the matter on the record, said that members of the U.S. military working on the ground in Syria were not in the immediate vicinity of the Turkish strikes but that the attack on a partner force with which they have worked closely had caused consternation among those troops. 

On Tuesday, Turkey also struck targets near Mount Sinjar in Iraq for the first time, widening its air campaign there. The PKK has been expanding its presence in the area since helping rescue thousands of Yazidis trapped there during the Islamic State’s 2014 rampage. 

Turkey and the government of Iraqi Kurdistan have repeatedly asked the PKK to leave. The tension has erupted into clashes in recent months, with several PKK-affiliated Yazidi fighters killed in confrontations with Kurdistan’s peshmerga forces. Local officials say this is distracting from pressing issues such as expelling the remaining Islamic State militants from the area and rebuilding. 

Khalil Adar, a spokesman for the PKK in Sinjar, said none of the group’s bases were targeted in the raid. Instead, in addition to the five peshmerga fighters killed, he said, locations of the PKK’s Yazidi affiliate were hit. 

Zerdasht Shengali, a spokesman for the Yazidi forces, said the Turkish strikes hit a radio station, a cultural center and a media center. One civilian was killed, he said. 

He called on all Kurdish parties to unite against Turkey. “We need to be one hand against our enemy,” he said. 

Turkey’s state news agency published aerial footage of what it said were the airstrikes in Iraq, showing large explosions at what appeared to be three locations, including a site with a tower that crumpled after being hit. 

Fahim reported from Istanbul. Aaso Ameen Shwan in Irbil, Missy Ryan in Washington and Zakaria Zakaria in Istanbul contributed to this report.