A Russian-backed Syrian government offensive against the country's last rebel enclave has caused one of the biggest waves of displacement in the nine-year-old war.
Weeks of intensive aerial bombardment and a bruising ground offensive have emptied entire towns in the northwestern region of Idlib and sent huge numbers fleeing northwards, closer to the Turkish border.
"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.
The exodus, which coincides with a biting winter, is one of the largest since the start in 2011 of a conflict during which more than half of the country's pre-war population of 20 million has been displaced.
"This latest displacement compounds an already dire humanitarian situation on the ground when over 400,000 people were displaced from the end of April through the end of August, many of them multiple times," Swanson said.
He said the UN was alarmed by the plight of more than three million people -- half of them displaced from their homes -- who live in Idlib province and surrounding areas.
Government troops and militia backed by Russian and other allied forces have in recent weeks ramped up the pressure on the last rebel-controlled pocket.
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 already hosts millions of Syrian refugees, is keen to stop another mass influx.
Some mountainous areas of northern Syria face sub-zero temperatures most winters and relief efforts have been stepped up once again.
'Disease outbreaks'
The World Health Organisation expressed concern on Monday that the conditions in the violence-plagued region were conducive to the outbreak of diseases.
The situation "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," WHO regional emergency director Rick Brennan said.
UN chief Antonio Guterres on Saturday urged all sides to halt the violence and allow for the necessary humanitarian effort to take place.
Syrian forces are conducting a two-pronged offensive on the Idlib enclave, from the south of the province where they have already reconquered much ground, and from the province of Aleppo to the west.
Further escalating tensions in the region, the Turkish and Syrian armies on Monday had their deadliest spat since Ankara sent troops to Syria in 2016.
On Monday, regime shelling of Turkish positions in Idlib left at least five Turkish soldiers and three civilians dead, Ankara said.
Retaliatory fire from Turkey killed at least 13 Syrian government troops, an incident that further tested an uneasy coordination between Russia and Turkey -- the two main foreign brokers in the Syrian conflict.
Under an agreement reached in Sochi last year, Russia was meant to prevent Damascus from launching a devastating operation in the densely-populated Idlib enclave.
Turkey's part of the deal was to contain the extremist groups running the region but instead the outfits it supports have been sidelined by the Hayat Tahrir al-Sham, an organisation led by members of Syria's former Al-Qaeda affiliate.