The United Nations has sounded the alarm over a severe humanitarian crisis unfolding in Syria's northwest, where a Russian-backed Syrian government push against the country's last rebel-held stronghold has forced more 500,000 people from their homes in two months.

"Since 1 December, some 520,000 people have been displaced from their homes, the vast majority - 80 percent - of them women and children," David Swanson, spokesman for the UN Office for the Coordination of Humanitarian Affairs, said on Tuesday.

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He added that the latest wave of displacement compounded "an already dire humanitarian situation" that saw more than 400,000 people displaced from the end of April through the end of August, many of them multiple times.

Swanson said the UN was alarmed by the plight of more than three million people - half of whom were transferred there en masse from other parts of Syria that were taken by government forces - who live in Idlib province and the surrounding areas.

Last Saturday, UNICEF, the UN's children agency, said the violence has forced 6,500 children to flee daily, and estimated that 1.2 million children "are in desperate need" amid short supplies of food, water and medicine.

According to the World Health Organization (WHO), more than 50 health facilities have suspended their services as of January 31.

"The current situation in northwest Syria - characterised by lack of access and medicine, insufficient hygiene, chaos and mass displacement - poses a significant risk of outbreaks of measles, diarrhoeal diseases and other diseases," said Rick Brennan, WHO's regional emergency director.

"Northwest Syria represents one of the world's most severe humanitarian crises, where civilians are suffering on an extraordinary level," he added, calling for "a renewed international commitment to bring an end to this protracted and devastating crisis".

Syrian government push against the country's last rebel-held stronghold has forced more 500,000 people from their homes in two months [File: Omar Haj Kadour/AFP]

Pushing refugees closer to Turkish border

In recent weeks, Syrian government troops and allied militias, backed by Russian and other forces, have ramped up the pressure on the last rebel enclave in the country.

They have retaken dozens of villages and some major towns - including the erstwhile rebel bastion of Maaret al-Numan - and are pushing northwards, sending displaced populations ever closer to the Turkish border.

Turkey, which hosts more than 3.5 million Syrian refugees and is wary of a new influx, has in recent days sent military vehicles, trucks and other reinforcements to the region,

Tensions rose on Monday as Ankara said at least seven Turkish soldiers and one civilian contractor working with the Turkish military were killed in shelling by Syrian government forces in Idlib.

Turkey retaliated by hitting 54 targets in Idlib and "neutralising" 76 Syrian government soldiers, the state-owned Anadolu agency quoted Defence Minister Hulusi Akar as saying.

The developments threaten to cause friction between Turkey and Russia, who have sought to coordinate their actions in Syria even though they back opposite sides in the conflict.