Nato has called on Syria and Russia to halt their air strikes following the killing of 33 Turkish soldiers in Syria.
The warning comes as Greece bolstered its border security as scores of migrants gathered at its border with Turkey seeking entry into Europe.
The European Union warned that the fighting in northern Syria could degenerate into open war and that the 27-nation bloc stands ready to protect its security interests.
Nato secretary-general Jens Stoltenberg said: “Allies condemn the continued indiscriminate air strikes by the Syrian regime and Russia in Idlib province. I call on them to stop their offensive, to respect international law and to back UN efforts for a peaceful solution.
“This dangerous situation must be de-escalated and we urge an immediate return to the 2018 ceasefire to avoid the worsening of the horrendous humanitarian situation in the region.”
Turkey’s 28 allies also expressed their condolences over the deaths, but no additional Nato support was offered during emergency talks, chaired by Mr Stoltenberg, between Nato ambassadors.
Apart from providing some aerial surveillance over Syria, Nato plays no direct role in the conflict-torn country, but its members are deeply divided over Turkey’s actions there, and European allies are worried about any new wave of refugees arriving.
The air strike by Syrian government forces marks the largest death toll for Turkey in a single day since it first intervened in Syria in 2016. It is a major escalation in a conflict between Turkish and Russia-backed Syrian forces that has raged since early February.
At least 54 Turkish troops have now been killed in Idlib over that time.
Omer Celik, spokesman for President Recep Tayyip Erdogan’s ruling party, said Turkey was “no longer able to hold refugees” following the Syrian attack – reiterating a long-standing warning from Mr Erdogan that his country could not cope with the arrival of more people fleeing the conflict.
Turkey hosts some 3.6 million Syrians and under a 2016 deal with the European Union agreed to step up efforts to halt the flow of refugees to Europe. Since then Mr Erdogan has repeatedly threatened to “open the gates” in several disputes with European states.
The DHA news agency reported that some 300 Syrians, Iranians, Iraqis, Moroccans and Pakistanis were gathering at the border with Greece, while others massed at beaches facing Greek islands off Turkey’s western coast.
A Greek police official said dozens of people had gathered on the Turkish side of the land border in Greece’s Evros region shouting “open the borders”. Greek police and military border patrols were deployed on the Greek side to prevent anyone trying to cross without authorisation.
A government official said Greece has beefed up border security “to the greatest possible degree … on land and at sea” and that Athens is in constant contact with the EU and Nato following the developments in Idlib.