After NATO member Turkey launched an assault aimed at curbing the Kurdish-led Syrian Democratic Forces (SDF), Stoltenberg acknowledged that Ankara had "legitimate security concerns" but called for a measured response.

"NATO has been informed by Turkish authorities about the ongoing operations in Northern Syria. It's important to avoid actions that may further destabilise the region, escalate tensions, and cause more human suffering," Stoltenberg said at a news conference in Rome with Italian Prime Minister Giuseppe Conte, in remarks released by his office.

"I count on Turkey to act with restraint and to ensure that any action it may take in northern Syria is proportionate and measured."

Turkish President Recep Tayyip Erdogan announced the start of "Operation Peace Spring", saying it aimed to stop a "terror corridor" emerging along Turkey's southern frontier.

The SDF was previously allied to the US, which used it to crush the Islamic State group. But Turkey says it is linked to Kurdish insurgents inside its own territory.

The assault has been strongly criticised by France and Germany, which warned it risked an IS resurgence, and Stoltenberg said the fight against the Islamists must not be endangered.

"We must not jeopardise the gains we have made together against our common enemy, ISIS," Stoltenberg said, using another acronym for the jihadist group.

"ISIS continues to pose a grave threat to the Middle East and North Africa, and to all our nations," he said.

Stoltenberg will visit Istanbul on Friday and he said he would raise the issue with Erdogan.