Video report by ITV News Correspondent Emma Murphy
Turkish President Recep Tayyip Erdogan has declared his country's borders with Europe are open, as thousands of migrants and refugees gathered at the frontier with Greece.
It comes after 33 Turkish troops were killed in an air strike on Thursday from Russian-backed Syrian forces.
The presidents of Turkey and Russia are in talks to defuse tensions following the attack.
The move by Turkey to open its border was seen in Greece as a deliberate attempt to pressure European countries - with tensions ratcheted up between Turkey and Syria.
In a speech in Istanbul on Saturday, Mr Erdogan said: "We will not close the gates to refugees.
He added: "The European Union has to keep its promises."
Under a 2016 deal, Turkey agreed to prevent refugees crossing the border to Europe in return for financial aid. It has since protested that the EU has failed to honor the agreement.
Greek Prime Minister Kyriakos Mitsotakis tweeted that no illegal crossings would be tolerated and that Greece was not to blame for the "tragic events in Syria".
He said "significant numbers" of migrants and refugees had gathered on the Turkish side of the border with Greece and called a meeting of top cabinet and military officials.
Police in Greece said an estimated 1,200 people had gathered late on Friday and periodically tried to push through. Military patrols have since been deployed to the border.
It was not clear whether Thursday's strike was carried out by Syrian or Russia jets, but Turkey blamed Syria’s government and Russia denied responsibility.
The air strike marked the deadliest day for the Turkish military since Ankara first entered the Syrian conflict in 2016.
The Turkish Defence Ministry released video on Friday showing retaliatory strikes on Syrian government positions.
In response to Thursday's attacked Turkey called an emergency Nato meeting where its 28 allies expressed their condolences over the deaths and urged de-escalation, but no additional Nato support was offered.
The European Union warned that the fighting in northern Syria could degenerate into open war and that it stood ready to protect its security interests.
The number of migrants at Turkey's Pazarkule border has been increasing since Thursday's air strikes. In response, Greece and Bulgaria have increased security at their borders with Turkey.
Apart from providing some aerial surveillance over Syria, Nato plays no direct role in the conflict, but its members are deeply divided over Turkey’s actions there.
Turkish President Recep Tayyip Erdogan, whose country already hosts more than 3.5 million Syrian refugees, has long threatened to "open the gates" for millions to flee to Europe unless more international support was provided.
The crisis stems from a Syrian government offensive that began on December 1 last year with Russian military support to retake Idlib province - the last opposition-held stronghold in Syria.
Turkey, the main backer of the Syrian opposition, has lost 54 soldiers this month, including the latest fatalities.
Thursday’s attack sharply raised the risk of direct military confrontation between Turkey and Russia, although Turkish officials blamed Syria for the air strike.
In their talks over the phone, Mr Erdogan and Russian President Vladimir Putin discussed implementing agreements in Idlib, the Kremlin said.
Fahrettin Altun, Mr Erdogan’s director of communications, said they had agreed to meet "as soon as possible".
Mr Erdogan talked with German chancellor Angela Merkel and US president Donald Trump in the wake of the attack.
Both called for the Syrian government and its supporters to stop their offensive and for a de-escalation to avoid a humanitarian crisis.
UN Secretary-General Antonio Guterres told reporters that he is talking with Russia and Turkey, appealing for a ceasefire in Idlib, but "we are not yet there".
Russia’s Defense Ministry said the Turkish troops that came under fire were deployed among "terrorist battle formations." Russian air forces did not carry out air strikes in the area, its statement said.
The Idlib offensive has triggered the largest single wave of displacement in Syria’s nine-year war - with 950,000 people fleeing to areas near the Turkish border for safety.
Turkish Foreign Ministry spokesman Hami Aksoy warned that the movement of migrants to the West could continue if the situation in Idlib deteriorated further.