At least 33 Turkish soldiers were killed in Syria's Idlib province after an air strike blamed on Damascus, prompting condemnation from Washington and a UN warning on the rapidly-rising risk of escalation, reports AFP.
Dozens more soldiers were injured and taken to Turkey for treatment, Rahmi Dogan, governor of Turkish Hatay -- bordering Syria -- said Friday.
The heavy losses in northwestern Idlib come after weeks of growing tensions between rebel supporter Ankara and Damascus ally Moscow.
"Without urgent action, the risk of even greater escalation grows by the hour," United Nations spokesman Stephane Dujarric said in a statement that reiterated Secretary General Antonio Guterres's call for an immediate ceasefire.
A US State Department spokesperson said Washington stands by its NATO ally Turkey and continues to call "for an immediate end to this despicable offensive by the Assad regime, Russia and Iranian-backed forces."
NATO chief Jens Stoltenberg urged de-escalation by all parties of "this dangerous situation", his spokesman said.
In a phone call with Turkish Foreign Minister Mevlut Cavusoglu, Stoltenberg "condemned the continued indiscriminate air strikes by the Syrian regime and its backer Russia in Idlib province", according to the spokesman.
Turkey has urged the Syrian regime to withdraw from Turkish observation posts in Idlib, while Moscow has accused Ankara of aiding "terrorists" in Syria.
Under a 2018 deal with Russia meant to bring calm to Idlib, Turkey has 12 observation posts in the region -- but several have come under fire from Syrian President Bashar al-Assad's forces.
Turkish President Recep Tayyip Erdogan hastily convened an emergency meeting in Ankara after the Idlib attack.
Erdogan's top press aide, Fahrettin Altun, said Turkey's military retaliated against all known positions of the regime after the air strike.
The latest attack means 53 Turkish security personnel have been killed in the province this month.
There was a flurry of diplomatic activity as Turkey's presidential spokesman Ibrahim Kalin spoke with US national security advisor Robert O'Brien, state news agency Anadolu reported without giving detail.
- More civilians killed -
Jihadists and Turkish-backed rebels on Thursday 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.
The counteroffensive could, however, be short-lived as Russian-backed Syrian troops continued to chip away at other parts of the rebel bastion, capturing 20 localities.
Seven civilians, including three children, were killed in regime and Russian bombardment of Idlib, according to the Syrian Observatory for Human Rights, adding to more than 400 such deaths since December.
The UN Security Council, where Moscow has systematically vetoed truce initiatives, met again Thursday amid growing concern Idlib is witnessing the nine-year-old war's worst humanitarian emergency.
State news agency SANA acknowledged there were "fierce clashes" between the army and "terrorist groups on the Saraqeb front".
An AFP correspondent accompanied the rebels into Saraqeb, where he found a ghost town of bombed out buildings.
The counterattack temporarily reverses one of the key gains of the government since its offensive against the country's last rebel enclave in December.
- Refugee fears -
The cash-strapped government had been keen to fully secure the M5, a highway that connects Syria's four main cities and passes through Saraqeb.