"I shared... my serious concerns about this ongoing operation and the risk of further destabilisation of the region," Stoltenberg told reporters in Istanbul, speaking alongside Turkish Foreign Minister Mevlut Cavusoglu.

"While Turkey has serious security concerns, we expect Turkey to act with restraint."

Stoltenberg emphasised that Turkey was an important part of the NATO alliance, but said the operation in Syria, which began on Wednesday, should not undermine gains against the Islamic State group.

"These gains must not be jeopardised. An imminent concern is that captured Daesh prisoners must not be allowed to escape," he said, using an alternative name for IS.

Turkey considers Kurdish militants in northern Syria a "terrorist" offshoot of Kurdish insurgents in its own territory.

It wants a 30-kilometre (20-mile) wide buffer zone along the border, which can also serve as an area to repatriate millions of Syrian refugees currently living in Turkey.