Eight humanitarian aid organisations have called for an immediate ceasefire in northwestern Syria, where hostilities have displaced half a million people in the past two months.

Russian-backed Syrian government forces have upped their deadly assault on the last major rebel bastion in the northwest since December, chipping away at its southern edge.

The violence in the rebel-held province of Idlib has forced 520,000 people out of their homes since the start of December, according to the United Nations, in one of the biggest upheavals in the nearly nine-year war.

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The aid groups - including the Norwegian Refugee Council, Save the Children, Care and the International Rescue Committee - labelled the situation a "humanitarian catastrophe".

They called for "an immediate cessation of hostilities in addition to immediate access to safety for the millions of civilians currently under fire".

The latest wave of people fleeing follows 400,000 others who were displaced by an earlier round of fighting in Idlib last year.

Many have fled north towards the border with Turkey, where camps are overcrowded and thousands more have instead set up haphazard tents in olive groves.

'Staggering' exodus

Jan Egeland, head of the Norwegian Refugee Council, warned that the new arrivals were running out of options as to where to go.

"Camps are hosting five times their intended occupancy and rental prices have skyrocketed in towns in the northwest," he said.

"We are calling on Turkey to let these terrified families seek safety either across the border or in areas Turkey controls in Syria."

Andrew Morley, the head of World Vision International, said children were sleeping in flooded fields, and some families were even burning their clothes to stay warm.

"The exodus of people is staggering, and tens of thousands more are joining them every day," he said.

Doctors have said cold weather, disease and a lack of shelter and medicine continues to threaten hundreds of thousands of civilians as they flee fighting in Idlib.

"People are facing a tragedy. For the last two weeks, it's been very, very cold. There is rain and mud, and influenza is spreading," said Wassim Zakaria, a doctor who works in a clinic in Idlib city that closed on Monday due to heavy bombardment.

Some are having to flee by foot, while many others are having to sleep in their cars, as Syrian and Russian warplanes bombard the highways leading north towards Turkey.

The World Health Organization said on Monday that the violence had forced 53 medical facilities in northwest Syria to close in January and warned of "critical health threats" to fleeing civilians.

Turkey-Russia tensions

Meanwhile, the UN Security Council will meet on Thursday for an emergency session on Syria following deadly clashes between the Turkish and Syrian military, diplomats said.

On Monday, eight Turkish military personnel were killed by Syrian government shelling in Idlib, prompting a Turkish military response in which Turkey said it struck more than 50 targets and killed 76 Syrian government troops. The Syrian government made no official comment on the reported deaths.

The diplomats said the meeting at the UN, which will be open to the public, was requested by the United States, France and the United Kingdom. On Wednesday, the US envoy for Syria, James Jeffrey, said his country is "very, very worried" about the escalation in Idlib.

The UN envoy for Syria, Geir Pedersen, is expected to report on the situation in Idlib, the diplomats said.

The Russian-backed Syrian government offensive on the northwestern region has drawn strong warnings from Turkey's President Recep Tayyip Erdogan. Turkey, which backs certain opposition groups in Idlib, has set up 12 observation posts in Idlib as part of a 2018 deal with Russia.

"If the Syrian regime will not retreat from Turkish observation posts in Idlib in February, Turkey will be obliged to take matters into its own hands," Erdogan said on Wednesday

"Turkey's air and land forces will move freely in all operation areas [in Syria] and in Idlib, and they will conduct operations if needed," he added.

Since hostilities intensified in the so-called "de-escalation zone" in Idlib on April 29, United Nations monitors have verified incidents in which 1,506 civilians, including 293 women and 433 children, were killed, UN human rights spokesman Jeremy Laurence said last month.

Syria's war has killed more than 380,000 people and displaced more than half the country's pre-war population since starting in 2011 with the repression of anti-government protests.