BRUSSELS — NATO on Friday called on Syria and Russia to halt their airstrikes following the killing of 33 Turkish soldiers in northeastern Syria, as scores of migrants gathered at Turkey’s border with Greece seeking entry into Europe.

Greece in turn bolstered border security, while 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.

Greek authorities suspended the operation of the customs post on the border with Turkey at Kastanies, Evros on Friday.

According to Athens-Macedonian News Agency (ANA) sources, the border crossing was closed after a large number of people, mainly refugees and migrants, gathered on the Turkish side of the border with the intention of coming to Greece.

Strong police forces have been sent to the location from early Friday and will gradually be further increased, in order to prevent the uncontrolled entrance of people into Greek territory.

“Allies condemn the continued indiscriminate air strikes by the Syrian regime and Russia in Idlib province,” Secretary-General Jens Stoltenberg said after chairing emergency talks between NATO ambassadors. “I call on them to stop their offensive, to respect international law and to back U.N efforts for a peaceful solution.”

“This dangerous situation must be de-escalated and we urge an immediate return to the 2018 cease-fire to avoid the worsening of the horrendous humanitarian situation in the region,” Stoltenberg said.

Turkey’s 28 allies also expressed their condolences over the deaths, but no additional NATO support was offered during the meeting.

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’s 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 in 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 longstanding warning from 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 Erdogan has repeatedly threatened to “open the gates” in several disputes with European states.

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 northeastern 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 authorization.

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.

The officials spoke on condition of anonymity as they were not authorized to speak to the press on the record.

Early Friday, broadcaster NTV showed images of dozens of people, carrying rucksacks, suitcases and plastic bags, crossing fields towards the Greek frontier.

Migrants walk to the Turkey-Greece border, in Pazarkule, Edirne, Friday, Feb. 28, 2020. NATO envoys were holding emergency talks Friday at the request of Turkey following the killing of 33 Turkish soldiers in northeast Syria, as scores of migrants gathered at Turkey’s border with Greece seeking entry into Europe. (AP Photo/Ergin Yildiz)

Near the Pazarkule border crossing with Greece, Turkish police stopped some 150 refugees about 1 kilometer (half a mile) from the border, preventing them from going further.

Meanwhile, EU foreign policy chief Josep Borrell warned that “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.”

In a tweet, Borrell called for the escalation around Idlib to “stop urgently,” and underlined that “the EU will consider all necessary measures to protect its security interests. We are in touch with all relevant actors.”

Turkey’s invasion of the north of the conflict-torn country — along with the criticism and threats of sanctions brandished by fellow allies at Ankara over the offensive — has come close to sparking a major crisis at NATO.

France in particular has tried to launch debate on what Turkey’s allies should do if Ankara requests their assistance under Article 5 of the Washington Treaty — which requires all allies to come to the defense of another member under attack — but that discussion has not happened.

Migrants arrive with a dinghy at the village of Skala Sikaminias, on the Greek island of Lesbos, after crossing the Aegean sea from Turkey, on Friday, Feb. 28, 2020. (AP Photo/Michael Varaklas)

The allies are extremely reluctant to be drawn into a conflict of Turkey’s making, and particularly because Erdogan has used up a lot of good will by testing his fellow NATO members’ patience for quite a while.

The Syria offensive comes on top of tensions over Turkey’s purchase of Russian-made S400 missiles, which threaten NATO security and the F-35 stealth jet. Erdogan also purged thousands of Turkish military officers following the failed coup in Turkey in 2016 and some have sought, and been granted, asylum in Europe.