Turkey announced on Sunday it had carried out air strikes against the bases of outlawed Kurdish militants across northern Syria and Iraq, which it said were being used to launch "terrorist" attacks on Turkish soil.
The raids in northern and northeastern Syria overnight, primarily against positions held by Syrian Kurdish forces, killed at least 31 people, British-based war monitoring group the Syrian Observatory for Human Rights said.
The offensive, codenamed Operation Claw-Sword, comes a week after a blast in central Istanbul that killed six people and wounded 81.
Turkey blamed the attack on the Kurdistan Workers' Party (PKK), which has waged a bloody insurgency there for decades and is designated a terror group by Ankara and its Western allies. The PKK has denied involvement in the Istanbul explosion.
"Air Operation Claw-Sword was successfully carried out, within the scope of our strategy to eradicate terrorism at its source and eliminate terror attacks against our people and security forces from northern Iraq and Syria," the defence ministry said in a statement.
"The hour of reckoning has come," the Turkish defence ministry tweeted early on Sunday, along with a photo of a plane taking off for a night operation.
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