The EU has received "reassurances" from the Turkish foreign minister that Ankara will stick to its side of a deal restricting migrant flows to Europe, the bloc's diplomatic chief said Friday.

EU foreign policy high representative Josep Borrell held talks by telephone with Turkey's Mevlut Cavusoglu after Ankara threatened to open the gates for refugees to flee to Europe - raising the spectre of a repeat of the 2015-2016 migrant crisis.

The warning came as Turkey sought international support after 33 of its soldiers were killed in an air strike in Syria. Ankara publicly blamed the deaths on the Damascus regime.

The Turkish threat to allow refugees free passage prompted Bulgaria and Greece to say they were tightening border security.

But Borrell said Cavusoglu had assured him Turkey stood by a 2016 deal with Brussels that dramatically cut the flow of migrants crossing the Aegean Sea to Greece.

"Spoke to Turkish Foreign Minister @MevlutCavusoglu. De-escalation remains key to effectively address challenges on ground. Human suffering and loss of life need to stop. Received reassurances that Turkey remains committed to Joint EU-Turkey Statement," Borrell tweeted.

Earlier, Borrell's spokesman Peter Stano said Turkish officials had not formally notified Brussels of any change to the deal, adding that the EU was assessing what was happening in terms of migrant movements.

Borrell also voiced deep alarm at the developments in Syria, which added to weeks of tensions between Turkey, which backs the rebels, and Assad's key ally Moscow.

"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.

The EU urged a rapid de-escalation and "will consider all necessary measures to protect its security interests," he said.

The EU however has only limited tools at its disposal to influence events on the ground in Syria and is regularly hamstrung by internal divisions on foreign policy issues.