“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 (people’s protection units) militia on Saturday in their enclave of Afrin, supporting Syrian opposition with airstrikes and ground troops.
The assault has raised fears among NATO member states that the fight against 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 defense support for Turkey “against missiles fired from Syria” but stressed it had no forces on the ground in the nation.
Turkish President Recep Tayyip Erdogan on Friday threatened to expand the offensive and vowed to “clean up” the Syrian city of Manbij.
Erdogan vowed in a speech in Ankara that Turkey would “continue our fight until there is no terrorist on our border.”
He said the operation would last until “we reach our goals,” adding: “Afterwards we will, as promised, clean up Manbij of terrorists.”
But Erdogan and Foreign Minister Mevlut Cavusoglu on Friday insisted Turkey was not intending to occupy Afrin and would return the region to its “real” owners.
According to Anthony Skinner, director of MENA at global risk consultancy Verisk Maplecroft, “direct military conflict” between Turkish and US forces is possible because of Erdogan’s threats to expand the campaign to Manbij.
“Turkish-US relations are teetering on the brink of a precipice,” Skinner added.
The EU has also expressed concern over the Turkish intervention in Syria, which is further complicating the war that has claimed more than 340,000 lives since 2011.