The alliance's ruling council held urgent talks at Turkey's request after Thursday's deadly air strike in Syria's northwestern Idlib province sent tensions soaring.

NATO Secretary General Jens Stoltenberg said allies had agreed to maintain measures already in place to bolster Turkey's air defences.

But he gave no hint of new steps beyond a general pledge to see what more could be done.

"NATO allies provide support for Turkey today. We augment their air defences, we have AWACS planes helping to patrol the skies and we also have port visits and provide support in other ways," he said.

"But allies are constantly looking into what more they can do to provide further support for Turkey."

A spokesman for the Turkish presidency earlier called for a no-fly zone to protect civilians in Idlib, where President Bashar al-Assad is seeking to wipe out the last rebel stronghold.

Assad's forces backed by Russian air strikes have since December clawed back chunks of the Idlib region, forcing close to a million people to flee their homes and shelters.

Stoltenberg condemned Damascus and Moscow and urged them to end the violence in Idlib.

- 'Stop the attacks' -

"We call on Russia and the Assad regime to stop the attacks, to stop the indiscriminate air attacks and also to engage and support UN-led efforts to find a lasting, political, peaceful solution to the crisis in Syria," he said.

The European Union's diplomatic chief Josep Borrell echoed the call for de-escalation, warning the crisis could spiral out of control.

"There is a risk of sliding into a major open international military confrontation. It is also causing unbearable humanitarian suffering and putting civilians in danger," he said.

NATO called the meeting on Friday under Article 4 of the alliance's charter under which any member can request talks if they believe their "territorial integrity, political independence or security" is threatened.

It is distinct from the alliance's Article 5 mutual self-defence pact, which refers to an attack on any member's territory.

The losses in Idlib come after weeks of growing tensions between Ankara and Damascus-ally Moscow. They bring to 53 the number of Turkish security personnel killed in the province this month.

Ankara has called talks under Article 4 a number of times in recent years -- twice in 2012, including after one of its jets was shot down by Syrian forces, and once in 2015 after a spate of terror attacks in Turkey.

After the 2012 incidents NATO agreed to deploy Patriot missile batteries in Turkey as a defensive measure.