More than 800,000 Syrians, mostly women and children, have fled their homes during a Russian-backed Syrian regime offensive to clear the opposition in northwest Syria since Dec. 1, a senior U.N. spokesperson said on Thursday.

The exodus is expected to continue, with thousands of people on the move as whole towns and communities flee to the safety of areas near the Turkish border, David Swanson told Reuters.

"Tragically, the number of people displaced – the vast majority of them women and children – has now surpassed 800,000," Swanson said.

Russian jets and Syrian artillery have pounded towns and villages since early December in a renewed assault backed by pro-Iranian militias and aimed at clearing the opposition.

"Residents are fleeing by the thousands in open trucks or by foot; often at night to escape detection despite the bitter cold temperatures," Swanson, the U.N. regional spokesman for Syria, told Reuters.

U.N. officials say the region is on the brink of a humanitarian catastrophe. Those on the run in Idlib and adjoining Aleppo province are joining close to 400,000 people who fled earlier bouts of fighting to the safety of camps near the Turkish border.

The latest offensive has brought the military campaign closer to heavily populated northern Idlib along the border, where nearly 3 million people are trapped, according to the U.N.

The border has become a refuge in recent years for people fleeing or being evacuated from previously opposition-held areas elsewhere in Syria.

Families fleeing airstrikes and advancing troops in Idlib province are sleeping in streets and olive groves, and burning toxic bundles of trash to stay warm in the biting winter weather, aid workers say.

AFP correspondents in Idlib have seen vast convoys of families with their mattresses stacked on trucks trekking across the war-torn province, amid an unusually bitter winter.

Many are sleeping in cars and in fields despite snowfall and sub-zero temperatures, raising fears of a major humanitarian catastrophe.

The U.N. Office for the Coordination of Humanitarian Affairs (OCHA) estimated that some 82,000 people are sleeping in the open air.

"Shelter is the most urgent need, as millions of people have been pushed into small areas not equipped to support that many people, especially during the cold winter," OCHA said.

It called the situation in Idlib "one of the worst" crises in the nine-year war.

The conflict in Syria has killed more than 380,000 people since it erupted in 2011 following the brutal repression of popular demonstrations demanding regime change.

More than half of Syria's population had already been displaced, but the latest regime offensive against the country's last rebel enclave has sent the numbers soaring again.

OCHA said some 142,000 people were displaced between Feb. 9 and Feb. 12 alone.

Aid organizations have called for a halt to the violence.