The World Health Organization has concerns that conditions in the violence-plagued region are conducive to the outbreak of diseases.
A Russian-backed Syrian regime offensive against the country's last opposition and rebel enclave has caused one of the biggest waves of displacement in the nine-year 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.
Assad's assault on Idlib, Aleppo
Assad regime forces, backed by Russian and other allied militias, have in recent weeks ramped up the pressure on the last opposition and rebel-controlled pocket.
They have retaken dozens of villages and some major towns, including the erstwhile rebel bastion of Maarat 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.
The World Health Organization 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.
'Syrian peace process not ended'
Amid continued carnage and hundreds of thousands of civilians fleeing northwestern Syria, the Astana and Sochi peace processes for Syria have not completely ended but are damaged, said Turkey’s foreign minister on Tuesday.
Russia can't make excuses for the Assad regime by saying “we can’t fully control the regime”, Mevlut Cavusoglu told reporters after an Asia Anew meeting in the Turkish capital Ankara.
Cavusoglu voiced concerns about the health of the Astana peace process, launched in 2017 in the former Kazakh capital, and the Sochi process, which in September 2018 established an oft-violated ceasefire for Idlib, northwestern Syria.
On Monday, an Assad regime attack in Idlib, near the area in which Turkey has carried out anti-terrorism operations, killed seven Turkish soldiers and one civilian contractor working with the Turkish military.
After the deadly attack, Turkey struck over 50 targets in retaliation and killed 76 Syrian regime soldiers, according to Turkish Defence Minister Hulusi Akar.