At least 33 Turkish soldiers were killed in Syria's Idlib province on Thursday in an aerial attack by Syrian “regime forces”, according to Governor Rahmi Dogan of Turkey's Hatay province, BBC reported.
Another 35 troops have been wounded.
A security meeting is being held at the presidential palace after the "nefarious attack against heroic soldiers in Idlib who were there to ensure our national security," according to a statement from Turkish director of communications Fahrettin Altun, CNN reported.
Turkey said it has carried out retaliatory attacks.
Nato Secretary General Jens Stoltenberg spoke by phone to Foreign Minister Mevlut Cavusoglu of Turkey, a key member of the military alliance.
Stoltenberg "condemned the continued indiscriminate air strikes by the Syrian regime and its backer Russia in Idlib province", his spokesperson was quoted as saying by the AFP news agency.
A spokesman for the US state department said in a statement: "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.
Meanwhile, UN Secretary General António Guterres expressed "grave concern" over the latest escalation, calling for an immediate ceasefire.
On February 27, Russia accused Turkey of violating the 2018 ceasefire by backing rebels with artillery fire.