Turkey's Operation Olive Branch in Afrin has alarmed NATO member states as they say it might disrupt the fight against Daesh in Syria.
NATO Secretary General Jens Stoltenberg said on Thursday that member state Turkey had a right to act in self-defence as Ankara presses a military operation against terrorist elements in northern Syria.
"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 Operation Olive Branch to remove the YPG/PKK and Daesh from Afrin in a bid to secure its border and bring peace to the region.
Afrin has been a YPG/PKK stronghold since July 2012 when the Assad regime in Syria abandoned the city to the terror group.
Ankara considers the YPG to be the Syrian branch of the PKK, which is a designated terrorist organisation in Turkey, the US and the EU.
But the operation has alarmed NATO member states as they say the fight against Daesh 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.