Nearly 1 million displaced in regime's Idlib offensive
That figure is 100,000 more than the UN had previously recorded.
"The crisis in northwest Syria has reached a horrifying new level," said Mark Lowcock, the UN head of humanitarian affairs and emergency relief.
He said the displaced were overwhelmingly women and children who are "traumatised and forced to sleep outside in freezing temperatures because camps are full. Mothers burn plastic to keep children warm. Babies and small children are dying because of the cold".
The Idlib region, including parts of neighbouring Aleppo province, is home to some 3 million people, half of whom have already been displaced from other parts of the country.
Syrian dictator Bashar al-Assad lost huge swathes of the country following an armed uprising after the brutal repression of peaceful protesters in 2011.
According to UN figures, the Syria war has caused the greatest refugee crisis in the world, with at least 5.5 million people fleeing the country and more than 6 million more displaced inside Syria.
According to the Syrian Observatory for Human Rights, more than 500,000 people have been killed since the start of the war in 2011.
Thousands are still missing and Human Rights Watch last week encouraged the authorities in Syria to investigate their fate.
Follow us on Facebook, Twitter and Instagram to stay connected