The Syrian army took control of the strategic northwestern crossroads town of Saraqeb on Saturday in the latest gain of a weeks-long offensive against the country's last major rebel bastion of Idlib. The advance came shortly after Turkey sent additional troops into the region and threatened to respond if any of its military observation posts in Idlib, set up under a 2018 truce, come under attack.

"Army units now exercise full control over the town of Saraqeb," state television reported, over footage of the town's streets, deserted after weeks of bombardment.

Since December, government forces have pressed a blistering assault against the Idlib region with Russian support, retaking town after town despite warnings from rebel ally Turkey to back off.

The violence has killed more than 300 civilians and sent some 586,000 fleeing civilians onto the roads, seeking relative safety nearer the Turkish border

The United Nations and aid groups have appealed for an end to hostilities, warning that scale of the exodus risks creating one of the worst humanitarian catastrophes of the nearly nine-year war.

But President Bashar al-Assad's forces have pressed on with their advance. Saraqeb is a strategic prize for the government it seeks to take back control of two major highways that meet in the town. The M5 motorway connects the capital to second city Aleppo. Running up from the Jordanian border in the south, it is Syria's longest highway.

The Britain-based Syrian Observatory for Human Rights said on Friday that government forces were in control of the whole extent of the M5 in Idlib province.

Only a 30 kilometre (20 mile) section in neighbouring Aleppo now remains in rebel hands. Saraqeb sits at the crossroads with the M4, the main east-west highway in Idlib which links Aleppo with the Mediterranean port city of Latakia, a government bastion. The M4 is still largely controlled by jihadists and their rebel allies.

Just a few months ago, Saraqeb counted some 110,000 residents, the Observatory says. But after weeks of deadly bombardment by the government and its ally Russia, large swathes of the town have been levelled and its inhabitants have fled.

To the north of the town, some three million people - half of them already displaced by the changing fortunes of the war - are huddling in what is left of rebel territory.

Some 50,000 fighters have holed up in the shrinking pocket, many of them jihadists but the majority allied rebels, according to Observatory estimates.

With Saraqeb recaptured, little more than half of Idlib province remains in rebel hands, along with slivers of neighbouring Aleppo and Latakia provinces.

Turkey, which already hosts some 3.7 million Syrian refugees, has called on the government to halt its campaign in Idlib, fearing a new influx across the border.

Tensions flared on Monday after a rare flare-up between government and Turkish forces in Idlib killed more than 20 people on both sides. Turkey has since sent additional troops into northwestern Syria, prompting Damascus to accuse Ankara of trying to hamper its advance.

According to Turkish state news agency Anadolu, Ankara has sent more than 350 vehicles with commandos and ammunition to reinforce its observation posts in Idlib since Friday.

A Turkish security source emphasised the troops' mission was solely to reinforce the 12 existing posts established under a 2018 deal with Russia aimed at preventing major fighting triggering a humanitarian crisis.

Turkey says three of its outposts - all in the southeastern part of Idlib - have been surrounded by the Syrian army. The Observatory said that all the Turkish reinforcements were deployed to outposts behind the front line north of Saraqeb.

"They have formed a buffer blocking further regime advances in Idlib," Observatory head Rami Abdul Rahman said.

Some Turkish troops were seen heading towards a military airport in the Taftanaz area, 16 kilometres (10 miles) north of Saraqeb, he said. Others were deploying to a military base south of Idlib city. Turkey has threatened to respond if any of its posts come under attack.

"Our observation posts in Idlib continue their duties and are capable of protecting themselves with the weapons and equipment they possess," the Turkish defence ministry said on Twitter. "In the event of a new attack, proper response will be given in the strongest manner, based on the right of self-defence."