"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 the Kurdish People's Protection Units (YPG) 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.

Cavusoglu slammed European governments who have criticised Turkey's offensive against the YPG.

"This is hypocrisy," he said. "Turkey tried diplomacy till the very last moment. Since it didn't work, we have to eliminate the threat."

They were asked about media reports that another NATO ally, Spain, might pull out its Patriot missiles from southern Turkey in response to the operation in Syria.

"We expect NATO allies to continue to provide support to Turkey because this is something that was agreed," said Stoltenberg.

"Turkey is on the frontline... We are here to protect not just Turkey but to protect ourselves," he added.

Cavusoglu said it underlined why Turkey had chosen to take delivery this summer of a Russian missile defence system, the S-400 -- a move which sparked significant concern among its NATO allies.

"All these debates and developments show that Turkey has to buy its own air defence systems," he said. "We are not going to beg endlessly... This is one reason why we bought the S-400."