Turkey retaliated against Syrian government forces after dozens of Turkish soldiers were killed in an air strike in Idlib province on Thursday.
The governor of Turkey's south-eastern province of Hatay, which sits on the Syrian border, said at least 33 soldiers were killed in the strike. Governor Rahmi Dogan said other seriously wounded soldiers were taken to hospitals in Turkey.
The Turkish armed forces retaliated with artillery fire and air attacks aimed at Syrian government targets, two Turkish security officials said on Friday.
"All known" Syrian government targets are under fire by Turkish air and land support units, Turkey's communications director, Fahrettin Altun, told state-run Anadolu news agency.
Turkey has decided to "respond in kind" to the attack by the Syrian government, Mr Altun said.
United Nations Secretary-General Antonio Guterres voiced "grave concern" about the escalation of fighting in north-west Syria and reiterated his call for a ceasefire.
"The Secretary-General is following with grave concern the escalation in northwest Syria and reports that dozens of Turkish soldiers have been killed by an airstrike," UN spokesman Stephane Dujarric said.
"The Secretary-General reiterates his call for an immediate ceasefire and expresses particular concern about the risk to civilians from escalating military actions. Without urgent action, the risk of even greater escalation grows by the hour."
In the wake of the bombing, a senior Turkish official said Ankara would no longer stop Syrian refugees from reaching Europe by land and sea, Reuters reported.
Turkish police, coast guard and border security officials have been ordered to stand down, the official said, in anticipation of the imminent arrival of refugees from Syria's Idlib where nearly a million have been displaced.
Turkish President Recep Tayyip Erdogan chaired an emergency security meeting late on Thursday to assess the latest developments in Idlib. The Turkish presidency said the meeting in Ankara lasted two hours but gave no details.
Omer Celik, spokesman for President Erdogan’s ruling party, said Nato should stand by Turkey’s side. Ankara recently called for US Patriot missiles to be deployed to defend its forces in Syria.
Foreign Minister Mevlut Cavusoglu spoke to Nato Secretary General Jens Stoltenberg, Anadolu reported.
The deaths are the largest number of fatalities among Turkish forces in a day since Ankara began sending troops into north-west Syria's Idlib province in recent weeks.
Turkey has posted thousands of soldiers and heavy military hardware in the province to support rebels looking to hold back an offensive by Syrian government and Russia aimed at taking back the rebel stronghold.
The latest fatalities come after the Turkish military death toll in the region had already reached 21 so far this month.
Turkish and Russian officials held talks in Ankara on Thursday. Two previous rounds in Ankara and Moscow have not produced a ceasefire deal.
Earlier on Thursday, Russia's Defence Ministry accused Turkey of illegally sending armed drones into Idlib province and providing artillery to support rebel fighters.
The ministry made the allegation after Syrian rebels backed by Turkish forces said they had recaptured the key town of Saraqeb, marking a first big push-back of a Syrian government offensive.
Russian state TV reported Turkish military specialists in the province were using shoulder-fired missiles to try to shoot down Russian and Syrian military aircraft.
The Russian ministry said Turkey was in breach of agreements it had made with Russia over Syria.
It said a Turkish strike drone had illegally entered Syrian airspace on Tuesday and been destroyed by Syrian government air defences while preparing a strike on government troops.