Istanbul, Feb 29 (AFP) The leaders of Russia and Turkey held crisis talks Friday after 33 Turkish soldiers died in an air strike in Syria, as Ankara ramped up pressure on Europe by threatening to flood in migrants.

The United States and United Nations urged an end to the Russian-backed Syrian offensive against rebel holdouts, but Turkey appeared intent on easing tensions with Moscow by pinning the blame squarely on President Bashar al-Assad's regime.

The flare-up raised fresh concerns for civilians caught up in the escalation of the horrific eight-year civil war, with the UN saying nearly a million people -- half of them children -- have been displaced in the bitter cold by the fighting since December.

Thirty-three Turkish troops were killed late Thursday in the air strike in the northwestern province of Idlib, in the biggest single loss of life by the Turkish military in years.

Russian President Vladimir Putin and Turkish President Recep Tayyip Erdogan spoke by telephone and looked to scale down tensions, with the Kremlin saying the two expressed "serious concern" about the situation.

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

He said the two leaders spoke of "the necessity to do everything" to implement a 2018 ceasefire that has since collapsed between the two countries in Idlib.

Erdogan may travel next week to Moscow for talks, Putin's spokesman Dmitry Peskov said. Before the killing of troops, Erdogan spoke of a meeting with Putin on March 5 but said it would also include the leaders of France and Germany.

US President Donald Trump condemned the attack on Turkish troops in a call with Erdogan and again urged Russia and Syria to halt the Idlib operation, the White House said.

Secretary of State Mike Pompeo called the attack "despicable and brazen" and said the US was looking at ways to support Turkey, a NATO ally that has recently drifted from the West.

A senior US official, while acknowledging that Turkey had blamed the Assad regime for the strike, said that Russia closely planned all operations with Syria.

"Russia is responsible for this offensive -- period," the official said in Washington on condition of anonymity.

The idea of the "pathetic, keelhauled, draftee Assad military forces fighting the Turks and some of the opposition forces... is laughable," he said.

Turkey said it retaliated by hitting more than 200 regime targets in drone and artillery bombardments.

The reprisals killed 45 Syrian soldiers in Idlib, according to a monitor, the Syrian Observatory for Human Rights. There was no confirmation from the Syrian government.

Rebel and Turkish fire also killed 10 fighters from Hezbollah, the Lebanese Shiite group backed by Iran that is supporting Assad, the Observatory said, adding that Russian strikes killed seven civilians.

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

At emergency talks on Friday, Russian Ambassador to the UN Vassily Nebenzia told the Security Council that Moscow was "ready to de-escalate with anyone who wants to" 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.

UN Secretary-General Antonio Guterres said the world body was planning to send a humanitarian mission there.

Diplomats said the mission to Idlib could start next week and include representatives of major UN agencies.

"The most pressing need is an immediate ceasefire before the situation gets entirely out of control," Guterres told reporters.

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

Turkey again called on the international community to establish a no-fly zone over Idlib, where Islamist fighters backed by Ankara pose the biggest obstacle to Assad seizing back control over all of Syria.

On Thursday, jihadists and Turkish-backed rebels had 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.

Erdogan's communications director Fahrettin Altun accused Assad on Twitter of "conducting ethnic cleansing" to drive millions out of Idlib, but said Turkey does not have the resources to accept more refugees.

Turkey has already taken in around four million Syrians and is wary of more arrivals in the face of growing popular discontent about their presence.

In a move seen as putting pressure on the West, Turkey threatened to go back on a deal with the EU and open the way for refugees to go into Europe.

"We will no longer keep the doors closed for refugees who want to go to Europe," a Turkish official told AFP.

In response, both Bulgaria and Greece said they were tightening border security as groups of migrants moved westwards across Turkey.

Greek border guards blocked hundreds of migrants from entering the country at the Kastanies border crossing in the northeast of the country as army trucks loaded with barbed wire raced past.

The EU called on Ankara to uphold its side of the 2016 migrant pact, in which the Europeans offered six billion euros in exchange for Turkey stemming the flow of migrants who had triggered a major backlash in the country. (AFP)

(This is an unedited and auto-generated story from Syndicated News feed, LatestLY Staff may not have modified or edited the content body)