WRAPUP 12-Turkey says it will let refugees into Europe after its troops killed in Syria

* Deadliest attack on Turkish troops in nearly 30 years

* Ankara's 2016 migrant deal with EU under threat

* U.N. chief urges all sides to step back from escalation

By Dominic Evans and Orhan Coskun

OUTSIDE SARAQEB, Syria/ANKARA, Feb 28 (Reuters) - Refugees in Turkey headed towards European frontiers on Friday after an official said the borders had been thrown open, a response to the escalating war in Syria where 33 Turkish soldiers were killed by Russian-backed Syrian government troops.

Moscow and Ankara traded blame over Thursday's strike in northwest Syria, the deadliest attack on Turkish forces in nearly 30 years. The U.N. Security Council held an emergency meeting to try to avert open conflict between Russia and NATO member Turkey.

U.S. President Donald Trump, in a phone call with Turkish President Tayyip Erdogan, condemned the attack and reaffirmed Washington's support for Ankara's efforts to avert a humanitarian disaster in Syria, a White House spokesman said.

The two leaders also said Syria and Russia must halt their offensive in northwest Syria, spokesman Judd Deere said.

U.N. Secretary-General Antonio Guterres described it as "one of the most alarming moments" of the nine-year-old Syrian war.

"The most pressing need is an immediate ceasefire before the situation gets entirely out of control," Guterres told reporters in New York. "In all my contacts with those involved, I have had one simple message: step back from the edge of escalation."

Turkey's neighbours Greece and Bulgaria, both European Union member states, vowed not to admit the migrants and reinforced their borders following Ankara's threat to reopen the frontier. It was closed under an accord between Turkey and the European Union that halted the 2015-16 migration crisis when more than a million people crossed into Europe by foot.

As news of Thursday's strike emerged, a senior Turkish official told Reuters the government had decided, effective immediately, not to stop Syrian refugees from reaching Europe "by land or sea," and that police and border guards had been stood down.

"All refugees, including Syrians, are now welcome to cross into the European Union," said the official, who spoke on condition of anonymity.

Within hours hundreds of migrants, some wearing face masks in an apparent attempt to guard against the coronavirus outbreak sweeping the world, began arriving on the European frontier in the early morning light.

"We heard about it on the television," said Afghan migrant Sahin Nebizade, 16, in a group packed into taxis on the outskirts of Istanbul.