"We have made enormous progress in the fight against ISIS...we must make sure that we preserve those gains," Stoltenberg said during a visit to Athens.

"We have to remember that we need to continue to stand together in our common fight against a common enemy, which is ISIS...ISIS are still present on the ground, in Iraq, Syria...in Afghanistan," he added.

The Turkish military, supported by Syrian proxies, launched the offensive against Kurdish-controlled areas in northeastern Syria on Wednesday, despite widespread international warnings.

Turkey says its operation is aimed at pushing back Syrian Kurdish forces, which it considers "terrorists", and establishing a "safe zone" with which to repatriate Syrian refugees.

Stoltenberg earlier this week urged Turkey to show "restraint" in its operation against Kurdish forces in Syria, warning that the fight against the Islamic State group should not be put at risk.

On Thursday he added that the operation should also avoid causing "more human suffering."

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.

Turkey's intervention has sparked international anger, raising fears of a new refugee crisis in northern Syria and concern that thousands of jihadists being held in Syrian Kurdish prisons could use the opportunity to escape.