ISTANBUL — The leaders of Russia and Turkey held crisis talks on Friday to try to scale down tensions after 33 Turkish soldiers were killed in a regime air strike in Syria.

The international community voiced fears of a rapidly-rising risk of escalation after the attack by Russian-backed Syrian forces in the province of Idlib, where President Bashar Assad is waging a bloody campaign to oust rebels from their last holdout.

Hours after the strike, Turkey warned it was opening the gates for refugees to flee to Europe, a move that could have major repercussions for its western neighbors and prompted Greece to double its border patrols.

The deadly bombardment added to weeks of mounting tensions between rebel supporter and NATO member Ankara and Damascus ally Moscow, and stoked international concerns about the plight of people living in the battleground province.

It was the highest single death toll since the Turkish army first intervened in Syria in 2016, and brought the total number of Turkish troops killed in Idlib this month to 53.

Turkey said it retaliated to Thursday's deaths by hitting more than 200 regime targets in drone and artillery bombardments.

The reprisals killed 16 Syrian soldiers, according to a monitoring group, but there was no immediate confirmation from Damascus.

Adding to the tensions, Moscow announced that two of its warships were transiting through the Bosphorus Strait in Istanbul in plain sight of the city.

But in a move to try to defuse the crisis, Russian President Vladimir Putin and his Turkish counterpart Recep Tayyip Erdogan spoke by phone on Friday.

"There is always room for dialogue," said Russian Foreign Minister Sergei Lavrov.

"The conversation was detailed and devoted to the necessity to do everything" to implement a ceasefire deal agreed in 2018 between the two countries to try to bring calm to Idlib.

Lavrov said Russia was ready to help improve the security of Turkish troops in Syria after the defense ministry said the slain soldiers had been among "terrorist groups" in Idlib.

The UN has repeatedly warned that the fighting in Idlib could potentially create the most serious humanitarian crisis since the start of the Syrian conflict in 2011.

But Russian vetoes, often backed by China, have chronically crippled UN action.

NATO held urgent talks on the crisis on Friday, and joined the UN and the US in calling for urgent de-escalation.

"There is a risk of sliding into a major open international military confrontation," said EU foreign policy chief Josep Borrell, echoing international alarm about the violence.

"It is also causing unbearable humanitarian suffering and putting civilians in danger."

Hours after the strike, Turkey threatened to go back on a deal with the EU and said it would open the way for refugees to go to Europe.

"We will no longer keep the doors closed for refugees who want to go to Europe," an official said on condition of anonymity.

Turkey, which is already home to around four million Syrian refugees, fears more people arriving in the country where there is growing popular discontent against their presence.

In response, Greece said it was tightening its border security "to the maximum level possible" and doubling border patrols.

The EU called on Ankara to uphold its side of the migrant pact which was aimed at stemming the flow of refugees and migrants towards Europe.

Turkey also Friday called on the international community to establish a no-fly zone over Idlib, where regime forces have since December clawed back chunks of the region, forcing close to one million people to flee their homes and shelters amid bitter cold.

In a series of tweets, Erdogan's communications director Fahrettin Altun accused Assad of "conducting ethnic cleansing" and seeking to drive millions of Syrians out of Idlib.

Groups of migrants have already moved towards western Turkey seeking to reach Greece, Turkey's private DHA news agency reported.

"Without urgent action, the risk of even greater escalation grows by the hour," UN spokesman Stephane Dujarric said, reiterating Secretary General Antonio Guterres's call for an immediate ceasefire.

The US State Department said Washington stood by Turkey and continues to call "for an immediate end to this despicable offensive by the Assad regime, Russia and Iranian-backed forces".

Elsewhere, jihadists and Turkish-backed rebels on Thursday re-entered Saraqeb, a key Idlib crossroads town they had lost earlier in February, reversing one of the main gains of the government's devastating offensive.

An AFP correspondent accompanied the rebels into Saraqeb, where he found a ghost town of bombed out buildings.

The counter-offensive could, however, be short-lived as the Russian-backed Syrian troops continued to chip away at other parts of the rebel bastion, capturing 20 localities.

On Friday, four members of a single family, two of them children, were killed in air strikes, the Britain-based Syrian Observatory for Human Rights said, adding to more than 400 such deaths since December.

State news agency SANA acknowledged there were "fierce clashes" between the army and "terrorist groups on the Saraqeb front". — AFP