At least 29 Turkish soldiers have been killed and several others injured in an airstrike by Syrian government forces in Idlib, a Turkish governor has said.

Turkish forces “responded in kind” almost immediately, retaliating with artillery fire on “all known” Syrian government targets, Turkey’s Anadolou news agency reported the state’s communications director Fahrettin Altun as saying.

The dangerous escalation marks the largest number of Turkish casualties in a single day since it sent thousands of troops to aid those resisting Bashar al-Assad‘s Russian-backed offensive, which began in December.

Idlib is the last major city held by opposition forces, and nearly 1 million residents have been forced to flee since the Syrian government’s aggressive bombing campaign began.

In anticipation of the imminent arrival of refugees from Idlib, Turkish border officials have been ordered to stand down and allow them to pass into Europe, a Turkish official told Reuters on Thursday.

Syrian regime bombards Idlib Show all 23 1 /23 Syrian regime bombards Idlib Syrian regime bombards Idlib Members of the Syrian Civil Defence, also known as the White Helmets, recover a wounded boy from the rubble of a building following a regime air strike on a vegetable market in Syria's last major opposition bastion of Idlib AFP via Getty Syrian regime bombards Idlib The regime air strikes killed at least nine civilians, striking bustling areas of Idlib city AFP via Getty Syrian regime bombards Idlib An injured woman waits to receive treatment at a makeshift hospital AFP via Getty Syrian regime bombards Idlib Syrian Civil Defence, also known as the White Helmets, searching through the rubble of a building AFP via Getty Syrian regime bombards Idlib A Syrian boy is evacuated AFP via Getty Syrian regime bombards Idlib A drone picture taken on January 15, 2020, shows members of the Syrian Civil Defence, also known as the White Helmets, searching through the rubble of a building at the site of a regime airstrike on Syria's last major opposition bastion of Idlib - Regime air strikes on Syria's last major opposition bastion killed at least nine civilians, striking bustling areas of Idlib city despite a fresh Russian-sponsored truce, a war monitor said. At least 20 other civilians were wounded in the raids that hit a vegetable market and repair shops in Idlib, capital of the jihadist-held province of the same name, said the Syrian Observatory for Human Rights. (Photo by Omar HAJ KADOUR / AFP) (Photo by OMAR HAJ KADOUR/AFP via Getty Images) OMAR HAJ KADOUR AFP via Getty Images Syrian regime bombards Idlib The wreckage of a car AFP via Getty Images Syrian regime bombards Idlib A Syrian boy cries as he is evacuated AFP via Getty Images Syrian regime bombards Idlib Rescuers search for survivors EPA Syrian regime bombards Idlib A drone picture taken on January 15, 2020, shows members of the Syrian Civil Defence, also known as the White Helmets, searching through the rubble of a building at the site of a regime airstrike on Syria's last major opposition bastion of Idlib - Regime air strikes on Syria's last major opposition bastion killed at least nine civilians, striking bustling areas of Idlib city despite a fresh Russian-sponsored truce, a war monitor said. At least 20 other civilians were wounded in the raids that hit a vegetable market and repair shops in Idlib, capital of the jihadist-held province of the same name, said the Syrian Observatory for Human Rights. (Photo by Omar HAJ KADOUR / AFP) (Photo by OMAR HAJ KADOUR/AFP via Getty Images) OMAR HAJ KADOUR AFP via Getty Images Syrian regime bombards Idlib A Syrian boy grimaces in pain after he was wounded in the airstrikes AP Syrian regime bombards Idlib A Syrian youth stands at the site of a regime air strike in Ariha town AFP via Getty Syrian regime bombards Idlib Rescuers search for survivors under the rubble of a collapsed building EPA Syrian regime bombards Idlib A Syrian walks on the rubble of a building following a regime air strike on Ariha town in Syria's last major opposition bastion of Idlib on January 15, 2020. - Regime air strikes on Syria's last major opposition bastion killed at least nine civilians Wednesday, striking bustling areas of Idlib city despite a fresh Russian-sponsored truce, a war monitor said. (Photo by Omar HAJ KADOUR / AFP) (Photo by OMAR HAJ KADOUR/AFP via Getty Images) OMAR HAJ KADOUR AFP via Getty Syrian regime bombards Idlib Firefighters try to extinguish flames EPA Syrian regime bombards Idlib Emergency services look for survivors AP Syrian regime bombards Idlib Emergency services look for survivors after airstrikes in the city of Idlib, Syria, Wednesday, Jan. 15, 2020. Syrian government warplanes struck a market and an industrial area Wednesday in the last territory in the hands of rebel groups in the country's northwest, killing at least 15 people, opposition activists said. (AP Photo/Ghaith Alsayed) Ghaith Alsayed AP Syrian regime bombards Idlib A boy wounded in airstrikes is treated in a hospital AP Syrian regime bombards Idlib Syrian White Helmet civil defense workers extinguish a burning car Syrian Civil Defense White Helmets via AP Syrian regime bombards Idlib A Syrian walks on the rubble of a building AFP via Getty Syrian regime bombards Idlib People walk past destruction by the government airstrikes in the town of Ariha, in Idlib province, Syria, Wednesday, Jan. 15, 2020. Syrian government warplanes struck a market and an industrial area Wednesday in the last territory in the hands of rebel groups in the country's northwest, killing at least 15 people, opposition activists said. (AP Photo/Ghaith Alsayed) Ghaith Alsayed AP Syrian regime bombards Idlib People walk past destruction by the government airstrikes in the town of Ariha, in Idlib province, Syria, Wednesday, Jan. 15, 2020. Syrian government warplanes struck a market and an industrial area Wednesday in the last territory in the hands of rebel groups in the country's northwest, killing at least 15 people, opposition activists said. (AP Photo/Ghaith Alsayed) Ghaith Alsayed AP Syrian regime bombards Idlib People look at destruction by the government airstrikes in the town of Ariha, in Idlib province, Syria, Wednesday, Jan. 15, 2020. Syrian government warplanes struck a market and an industrial area Wednesday in the last territory in the hands of rebel groups in the country's northwest, killing at least 15 people, opposition activists said. (AP Photo/Ghaith Alsayed) Ghaith Alsayed AP

This has not been confirmed by the government in Turkey, which hosts some 3.7 million Syrian refugees and in 2016 struck a multibillion-euro deal with the European Union to stem the influx of migrants into the bloc.

More than 900,000 people in Idlib have fled their homes since December, with some 500,000 estimated to be children.

Most are homeless and sleeping in freezing conditions at camps and makeshift shelters in the region. Many have resorted to burning spare clothes and garbage to stay warm.

Offensives by Syrian government forces are shifting the front lines closer to these densely populated areas, leading the UN deputy regional humanitarian coordinator for Syria to warn last week that there will be “a bloodbath” if Mr Assad’s forces push further into the province.

“We’re going to see a massacre on a scale that has never been seen in this entire war,” Mark Cutts told Sky News on Thursday.

Mr Assad’s forces, supported by relentless Russian airstrikes, have pushed hard in recent months to retake the last large rebel-held region in northwest Syria after nine years of war that has displaced millions and killed hundreds of thousands.

Turkey’s president Recep Tayyip Erdogan has threatened to mount a major counter-offensive against Mr Assad’s forces unless they pull back within the next two days to ceasefire lines agreed in 2018.

Two Turkish officials said he held an emergency meeting in Ankara with staff after the airstrike on Thursday, but the presidency would not release any details. Turkey previously suffered 21 casualties in February.

The airstrike came after Turkey-backed Syrian opposition fighters retook a strategic northwestern town from government forces on Thursday, opposition activists said, cutting a key highway just days after the government reopened it for the first time since 2012.

Despite losing the town of Saraqeb, Mr Assad’s forces now control almost the entire southern part of Idlib province after capturing more than 20 villages on Thursday, state media and opposition activists said.

While the fighting raged, Turkish and Russian officials concluded two days of talks in Ankara. Two previous rounds of negotiations in Ankara and Moscow have not yielded a sought ceasefire deal.

Hours earlier, Russian state television claimed Turkish military specialists were using shoulder-fired missiles to try to shoot down Russian and Syrian military aircraft over Idlib, a development that, if confirmed, would mark another serious escalation of the conflict.