"Civilians at all times must be protected," foreign ministry spokesman Christofer Burger told a government briefing, describing as "extremely worrying" reports of civilian victims in the Turkish air raids.

"We call on Turkey to react proportionally and to respect international law," he added.

The Turkish raids in northern and northeastern Syria killed at least 31 people, said the British-based monitoring group the Syrian Observatory for Human Rights on Sunday. The raids were mainly against positions held by Syrian Kurdish forces.

The offensive, codenamed Operation Claw-Sword, comes a week after a bombing in Istanbul killed six people and wounded 81. Turkey blamed the attack on the Kurdistan Workers' Party (PKK).

The PKK has waged a bloody insurgency for decades and is designated a terror group by Ankara and its Western allies. But it has denied involvement in the Istanbul explosion.

President Recep Tayyip Erdogan on Monday said that Turkey could launch a ground operation in Syria.

Burger said Germany urged "Turkey and all other participants to do nothing that would aggravate the already tense situation in northern Syria and Iraq."