State Department spokesman Mark Toner on Tuesday expressed "deep concern" over the airstrikes, telling reporters that they had not been authorized by the international coalition fighting the so-called "Islamic State" (IS) militant group.

At least 20 Kurdish fighters were killed earlier on Tuesday when Turkish jets carried out bombings in northeast Syria and northern Iraq, part of an ongoing effort to eliminate what Ankara calls "terrorist havens" with ties to the banned Kurdistan Workers' Party (PKK).

Toner said the State Department had spoken with Ankara directly about the strikes, which "led to the unfortunate loss of life of our partner forces in the fight against" IS.

The strikes in Syria targeted members of the Kurdish People's Protection Units (YPG), a faction of the US-backed Syrian Democratic Forces (SDF), which have proven integral in the fight against IS. Ankara has strongly opposed Washington's support of the group.

'Obliged to take measures'

Following the bombings, the YPG said a media center, a radio station, communications facilities and military institutions had been hit.

"As a result of the barbaric strikes by the Turkish warplanes at dawn today against the YPG center...20 fighters were martyred and 18 others wounded, three of them critically," said the group's spokesman.

But Turkish President Recep Tayyip Erdogan has defended the move.

"We are obliged to take measures. We must take steps. We shared this with the US and Russia and we are sharing it with Iraq as well," Erdogan told the Reuters news agency. "It is an operation that [Iraqi Kurdistan President Massoud] Barzani has been informed about."

Toner said Washington recognized Erdogan's concerns about the PKK but said such strikes harmed efforts to combat IS.

blc/cmk (AFP, Reuters)