At least 33 Turkish soldiers have died in a Syrian government attack in opposition-held north-western Syria, in a major escalation of the conflict.
Turkey, which backs the opposition, says it hit 200 government targets in response, “neutralising” 309 soldiers.
Russia, Syria’s key military ally, says Turkish troops were attacked in Idlib province by Syrian forces while operating alongside jihadist fighters.
It denies its own forces were involved in fighting in the Balyun area.
Government forces, supported by Russia, have been trying to retake Idlib from jihadist groups and Turkish-backed rebel factions based there. The air strike came after the opposition retook the key town of Saraqeb, north-east of Balyun.
Idlib is the last Syrian province to remain in opposition hands.
Reports suggest Turkey, a key member of the Nato alliance, may be relaxing its border controls to allow Syrian refugees to seek refuge in the EU.
What do we know about the attack?
“Thirty-three of our soldiers were martyred as a result of the air strike… by the forces of the [Bashar al-]Assad regime,” said Rahmi Dogan, the governor of Turkey’s neighbouring Hatay province.
Those wounded were being treated in hospital in Turkey but none were in a critical condition, he added.
After President Recep Tayyip Erdogan held an urgent top-level security meeting in Ankara, Turkish forces began conducting ground and air strikes.
Defence Minister Hulusi Akar said that in addition to the casualties inflicted, five Syrian government helicopters, 23 tanks, 23 howitzers, and two air defence systems had been destroyed.
According to the Russian defence ministry, the Turkish soldiers had been killed in a “bombardment” while operating alongside “terrorists” in the Balyun area where, it said, fighters from the Hayat Tahrir al-Sham alliance (formerly the Nusra Front) were attacking Syrian government forces.
Russia said it was in constant contact with Turkey to ensure Turkish troops were not targeted in Idlib and had not been informed that Turkish forces were active at Balyun.
But Mr Akar insisted the Russians had been informed about the locations of Turkish troops and said no armed groups had been present near the soldiers who were attacked. He also said ambulances had been hit in the attack.
An unconfirmed Russian media report says two Russian warships equipped with cruise missiles are being sent towards the Syrian coast.
President Erdogan wants Syrian government forces to pull back from positions where Turkey has set up military observation posts and earlier threatened to attack them if they did not halt their advance.
But Syria’s government and Russia have rejected his demand to pull back to ceasefire lines agreed in 2018. Russia has also accused Turkey of violating the 2018 ceasefire by backing rebels with artillery fire.
In reaction to the crisis:
Nato Secretary General Jens Stoltenberg spoke by phone to Turkish Foreign Minister Mevlut Cavusoglu and “condemned the continued indiscriminate air strikes by the Syrian regime and its backer Russia in Idlib province”, and Turkey is meeting its Nato partners on Friday
A spokesman for the US state department 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. We are looking at options on how we can best support Turkey in this crisis”
UN Secretary General António Guterres expressed “grave concern” over the latest escalation, calling for an immediate ceasefire.