"All nations have the right to defend themselves, but this has to be done in a proportionate and measured way," Stoltenberg said in a statement issued by his office.

Turkey launched an offensive against the Kurdish YPG militia on Saturday in their enclave of Afrin, supporting Syrian rebels with air strikes and ground troops.

Ankara views the YPG as a terror group linked to the outlawed Kurdistan Workers' Party (PKK) inside Turkey. The PKK is blacklisted by Ankara and its Western allies.

But the assault has raised fears among NATO member states that the fight against Islamic extremists in Syria might be impacted by Turkey's push.

"Turkey has also briefed Allies at NATO this week on their operation in northern Syria," Stoltenberg said.

"Turkey is one of the NATO nations that suffers the most from terrorism."

The NATO chief added that the alliance was providing air defence support for Turkey "against missiles fired from Syria" but stressed it had no forces on the ground in the war-torn nation.