Hundreds of thousands of people are facing a “massacre” on a scale never seen before in Syria as government forces advance into the last major rebel stronghold in the country, a senior UN official has warned.

Almost 900,000 people have been displaced since December because of a Russian-backed offensive by the army of Syria’s president, Bashar al-Assad, leaving many people homeless and forced to sleep in freezing temperatures.

Mark Cutts, the UN deputy regional humanitarian coordinator for Syria, has warned that there will be “a bloodbath” if Mr Assad’s forces push further into the northwest Idlib province.

“There are about a million displaced people living in camps and makeshift shelters in that area and if the shelling and airstrikes move any further in that area we’re going to see a bloodbath,” Mr Cutts told Sky News on Thursday.

“We’re going to see a massacre on a scale that has never been seen in this entire war.”

Russian warplanes struck again at rebel-held towns in the country’s northwest on Thursday, as Turkish artillery supported insurgent attacks elsewhere.

Syrians forced to flee Maarat al-Numan as Assad forces move in Show all 8 1 /8 Syrians forced to flee Maarat al-Numan as Assad forces move in Syrians forced to flee Maarat al-Numan as Assad forces move in Displaced Syrian children sit in a small truck, as they head to the area where their family and others will settle in, after fleeing violence of Maarat al-Numan town EPA Syrians forced to flee Maarat al-Numan as Assad forces move in epa08100426 Displaced Syrian children look on as adults set up tents, after fleeing violence of Maarat al-Numan town, near the Turkish border, Northern Syria, 28 December 2019 (issued 03 January 2020). According to various media reports, some 30,000 people have left Maarat al-Numan, considered as the last strong hold of rebels in northern Syria. The number of the displaced, allegedly reached this figure by 24 December, almost emptying the devastated city from its inhabitants, who fled air strikes and heavy shelling. The Internally displace people fled towards the Turkish border, with many of them building unofficial camps in difficult terrain and wintery weather, reports said. EPA/YAHYA NEMAH YAHYA NEMAH EPA Syrians forced to flee Maarat al-Numan as Assad forces move in epa08100437 A Displaced Syrian child sits in a small truck, upon arriving to the area where her family and others will settle in, after fleeing violence of Maarat al-Numan town, near the Turkish border, Northern Syria, 28 December 2019 (issued 03 January 2020). According to various media reports, some 30,000 people have left Maarat al-Numan, considered as the last strong hold of rebels in northern Syria. The number of the displaced, allegedly reached this figure by 24 December, almost emptying the devastated city from its inhabitants, who fled air strikes and heavy shelling. The Internally displace people fled towards the Turkish border, with many of them building unofficial camps in difficult terrain and wintery weather, reports said. EPA/YAHYA NEMAH YAHYA NEMAH EPA Syrians forced to flee Maarat al-Numan as Assad forces move in epa08100448 Displaced Syrian children look on from inside a tent upon arriving to the area where her family and others will settle in, after fleeing violence of Maarat al-Numan town, near the Turkish border, Northern Syria, 28 December 2019 (issued 03 January 2020). According to various media reports, some 30,000 people have left Maarat al-Numan, considered as the last strong hold of rebels in northern Syria. The number of the displaced, allegedly reached this figure by 24 December, almost emptying the devastated city from its inhabitants, who fled air strikes and heavy shelling. The Internally displace people fled towards the Turkish border, with many of them building unofficial camps in difficult terrain and wintery weather, reports said. EPA/YAHYA NEMAH YAHYA NEMAH EPA Syrians forced to flee Maarat al-Numan as Assad forces move in epa08100436 Displaced Syrians carry rocks as they clear an area to set up tents, after fleeing violence of Maarat al-Numan town, near the Turkish border, Northern Syria, 28 December 2019 (issued 03 January 2020). According to various media reports, some 30,000 people have left Maarat al-Numan, considered as the last strong hold of rebels in northern Syria. The number of the displaced, allegedly reached this figure by 24 December, almost emptying the devastated city from its inhabitants, who fled air strikes and heavy shelling. The Internally displace people fled towards the Turkish border, with many of them building unofficial camps in difficult terrain and wintery weather, reports said. EPA/YAHYA NEMAH YAHYA NEMAH EPA Syrians forced to flee Maarat al-Numan as Assad forces move in epa08100444 The muddy plastic shoes of a little girl upon arriving to the area, where her family and others will settle in, after fleeing violence of Maarat al-Numan town, near the Turkish border, Northern Syria, 28 December 2019 (issued 03 January 2020). According to various media reports, some 30,000 people have left Maarat al-Numan, considered as the last strong hold of rebels in northern Syria. The number of the displaced, allegedly reached this figure by 24 December, almost emptying the devastated city from its inhabitants, who fled air strikes and heavy shelling. The Internally displace people fled towards the Turkish border, with many of them building unofficial camps in difficult terrain and wintery weather, reports said. EPA/YAHYA NEMAH YAHYA NEMAH EPA Syrians forced to flee Maarat al-Numan as Assad forces move in epa08100447 Displaced Syrian children sit in a small truck next to bed covers and floor spreads, upon arriving to the area where their family and others will settle in, after fleeing violence of Maarat al-Numan town, near the Turkish border, Northern Syria, 28 December 2019 (issued 03 January 2020). According to various media reports, some 30,000 people have left Maarat al-Numan, considered as the last strong hold of rebels in northern Syria. The number of the displaced, allegedly reached this figure by 24 December, almost emptying the devastated city from its inhabitants, who fled air strikes and heavy shelling. The Internally displace people fled towards the Turkish border, with many of them building unofficial camps in difficult terrain and wintery weather, reports said. EPA/YAHYA NEMAH YAHYA NEMAH EPA Syrians forced to flee Maarat al-Numan as Assad forces move in epa08100441 Displaced Syrians clear an area to set up tents, after fleeing violence of Maarat al-Numan town, near the Turkish border, Northern Syria, 28 December 2019 (issued 03 January 2020). According to various media reports, some 30,000 people have left Maarat al-Numan, considered as the last strong hold of rebels in northern Syria. The number of the displaced, allegedly reached this figure by 24 December, almost emptying the devastated city from its inhabitants, who fled air strikes and heavy shelling. The Internally displace people fled towards the Turkish border, with many of them building unofficial camps in difficult terrain and wintery weather, reports said. EPA/YAHYA NEMAH YAHYA NEMAH EPA

The front lines are moving closer to densely populated areas, with bombardments increasingly affecting camps for the internally displaced (IDPs).

More than 500,000 of those displaced are thought to be children.

Families are sleeping by roads and in olive groves, burning garbage to stay warm.

The warning came as Mark Lowcock, the UN’s humanitarian affairs chief, told the UN Security Council that “the unfolding humanitarian catastrophe” in Idlib had “overwhelmed” efforts to provide aid.

“Many are on foot or on the backs of trucks in below-freezing temperatures, in the rain and snow,” Mr Lowcock said.

“They are moving into increasingly crowded areas they think will be safer. But in Idlib, nowhere is safe.”

He added: “I am getting daily reports of babies and other young children dying in the cold.”

Antonio Guterres, the UN secretary-general, said on Tuesday that he was “alarmed by the rapid deterioration of the humanitarian situation” in the country.

“In these dramatic circumstances, a ceasefire is essential and international humanitarian law must be upheld,” he said.

Filippo Grandi, the UN high commissioner for refugees, has called for an end to the fighting and appealed to all parties to “respect the rules of war”.

“Thousands of innocent people cannot pay the price of a divided international community, whose inability to find a solution to this crisis is going to be a grave stain on our collective international conscience,” Mr Grandi said.

“As humanitarian agencies, we are striving to save lives, but the space for these efforts is shrinking. In the face of such suffering, humanitarian aid alone cannot be the answer.”

In September, Russia, which supports Mr Assad’s government, blocked a demand by the UN National Security Council for a truce in northwest Syria because it did not include an exemption for military offensives against UN-blacklisted militant groups.

Although the UN has called on Russia and Turkey, which supports opposition forces in Syria, to work towards de-escalation in the country, talks between the two countries in recent days have not produced results.

Turkish officials sounded more optimistic on Thursday and one said the latest round had not been “completely without a result”.

The United States, United Kingdom, Germany and others have emphasised that three-way negotiations between Turkey, Russia and Iran – known as the Astana talks, which previously led to a de-escalation zone in Idlib, are not working.

Christoph Heusgen, Germany’s ambassador to the UN, has urged Russia to stop supporting the Syrian government.

“If you tell the Syrians that there is no longer military support to the Syrian regime, they will have to stop the onslaught on their own population,” Mr Heusgen said.

On Monday, Mr Assad promised that his government’s offensive would not end with the defeat of rebel forces in Aleppo.

“This liberation does not mean the end of the war, and does not mean the end of the schemes nor the end of terrorism or the surrender of enemies,” the Syrian president said in a televised address.