Turkish air strikes overnight killed at least 31 people in northern Syria, primarily in positions held by Syrian Kurdish forces, a Britain-based war monitoring group said Sunday.

Nearly 25 strikes hit Raqa, Hassakeh and Aleppo provinces, killing 18 members of the Kurdish-led Syrian Democratic Forces, 12 members of Syria's military and one journalist, said the Syrian Observatory for Human Rights.

The autonomous Kurdish administration in northeastern Syria meanwhile gave a toll of 29 dead -- including 11 civilians, 15 fighters aligned with Syria's military, two silo guards and one Kurdish fighter.

Turkey said the strikes were against bases of outlawed Kurdish militant group the Kurdistan Workers' Party (PKK) across northern Syria and Iraq, which it said were being used to launch "terrorist" attacks on Turkish soil.

The offensive, codenamed Operation Claw-Sword, comes a week after a blast in central Istanbul that killed six people and wounded 81.

The strikes in Syria mainly targeted the northern city of Kobane and its surroundings near the Turkish border, including grain silos near Al-Malikiyah in the northeast and a power plant in an area under the control of the SDF.

"These attacks by the Turkish occupation state will not go unanswered," the SDF said in a statement. "At the appropriate time and place, we will respond forcefully and effectively."