(CNN) At least 33 Turkish soldiers were killed in Syria's Idlib province on Thursday in an aerial attack by Russian-backed Syrian regime forces, Turkish officials said, raising the specter of full-blown conflict.
Thirty-five soldiers were injured in the attack and have been evacuated to hospitals in Turkey, according to officials.
The attack comes days before a Turkish deadline on Syrian government troops to withdraw from areas recaptured in recent months from rebels in northwest Syria.
Turkey, which backs some Syrian anti-government fighters, condemned the Syrian attack as a "nefarious attack against heroic soldiers in Idlib who were there to ensure our national security."
Turkish Defense Minister Hulusi Akar said the attack happened "despite coordination with the officials of the Russian Federation who were on the ground when this attack happened." Akar said ambulances were hit during the strikes.
The Syrian government has not commented on the Turkish claim. The Russian Defense Ministry denied that its air force carried out strikes in the area of Idlib where the Turkish soldiers were located. Moscow said Turkish forces were "located near the areas where terrorist groups were situated" and then "came under fire from Syrian forces."
The European Union High Representative for Security Policy, Josep Borrell Fontelles, warned of "a risk of sliding in a major open international military confrontation," in a tweet on Friday.
"Ongoing escalation around needs to stop urgently," Fontelles said.
The North Atlantic Council (NATO) will meet Friday over the attack, after Turkey, a member state, requested consultations.
Turkey has retaliated in an effort to "revenge our martyred heroic soldiers," a Turkish government statement said.
A spokesperson for the State Department said the United States is "very concerned."
"We are in contact with Turkish authorities to confirm these developments and to have more clarity on the current situation on the ground," the spokesperson said.
"We stand by our NATO Ally Turkey and continue to call for an immediate end to this despicable offensive by the Assad regime, Russia and Iranian-backed forces."
Turkish soldiers are in the last rebel-held area of Syria as part of a 2018 de-escalation agreement between Ankara and Moscow. The Syrian government, backed by Russia, has mounted an aggressive air campaign against rebels in Idlib in recent weeks. Russia has rejected calls for a ceasefire.
Hundreds of thousands of people have fled the last opposition-held territory in Syria in the last two months, per United Nations figures, in the wake of an air campaign and swift ground offensive by the Syrian regime and its Russian backers.
Syrian government forces have targeted multiple schools, converted into shelters, and hospitals in recent days, according to activist reports and videos.
"We think we are safe and then the warplanes come and take everything from us," said one media activist, Hiba, in a social media video from one of the damaged schools.
Tens of thousands are still on the move, and nearly 700,000 of the displaced are women and children, the UN said.