Turkey warned on Friday it was opening the gates for refugees to flee to Europe after an airstrike blamed on Damascus killed 33 Turkish soldiers 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 al-Assad is waging a bloody campaign to oust rebels from their last holdout.

The deadly bombardment has added to weeks of growing tensions between rebel supporter and NATO member Ankara and Damascus ally Moscow.

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

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

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

Turkey retaliated to Thursday's strike by hitting Damascus regime targets "from the air and ground," President Recep Tayyip Erdogan's communications director Fahrettin Altun said.

The United States condemned the Syrian action, and NATO said it would hold urgent talks on the crisis on Friday.

In a move that could have major repercussions for Turkey's neighbours to the west, Ankara 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 told AFP on condition of anonymity.

Risk of 'bloodbath'

Turkey also 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.

The United Nations said Monday that the latest fighting was coming "dangerously close" to encampments of the displaced, risking an imminent "bloodbath".

Turkey, which is already home to around 3.6 million Syrian refugees, fears more people arriving in the country where there is growing popular discontent against their presence.

In a series of tweets, Altun accused Assad of "conducting ethnic cleansing" and seeking to drive millions of Syrians out of Idlib. 

"These people will try to escape to Turkey and Europe. Already hosting close to 4 million refugees, we do not have the capacity and resources to allow entry to another million," he wrote.

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

'Dangerous situation'

The international community voiced alarm over the latest violence in Idlib.

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

NATO chief Jens Stoltenberg urged de-escalation by all parties of "this dangerous situation" and condemned the "indiscriminate airstrikes", in a phone call with Turkish Foreign Minister Mevlut Cavusoglu.

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

Under a 2018 deal with Russia meant to bring calm to Idlib, Turkey has 12 observation posts in the Idlib region -- but several have come under fire from Assad's forces.

In its first response to the Turkish deaths, Russia's defence ministry said the troops were among "terrorist groups" and that they had not communicated their presence in the area.

Thursday's attack brings to 53 the number of Turkish security personnel killed in Idlib so far this month.

More civilians killed

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.

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

Seven civilians, including three children, were killed in regime and Russian bombardment of Idlib, according to the Syrian Observatory for Human Rights, 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".

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

The counter-attack temporarily reverses one of the key gains of the government since its offensive against the country's last rebel enclave in December.

The cash-strapped government had been keen to fully secure the M5, a highway that connects Syria's four main cities and passes through Saraqeb.

The Syrian Observatory said the airstrikes were carried out by Russia, heavily criticised by the West for the high civilian death toll from its bombing campaign.

State media accused the "terrorists" of launching car bombings and other suicide attacks against government forces attempting to retake the town.

It said the army had inflicted heavy losses on the attackers, despite the military support it said they had received from Turkey.

Some 950,0000 civilians have fled the government offensive, raising fears in Ankara of a new influx of refugees.

More than half a million of those displaced since December are children, tens of thousands of whom are sleeping rough in northern Syria's harsh winter.

Erdogan had vowed on Wednesday that Ankara would not take the "smallest step back" in the standoff with Damascus and Moscow over Idlib.