Tens of thousands of civilians fled their homes in northwestern Syria last week in the face of intensified bombardment of rebel territory by pro-government forces, the United Nations said on Friday.
Between 15 and 19 January, more than 38,000 people fled violence in the rebel-held west of Aleppo province, which neighbours the main opposition bastion of Idlib, the UN humanitarian affairs office (OCHA) said.
OCHA spokesman David Swanson said the displaced were heading north into territory controlled by Turkish-backed rebel forces or west into jihadist-dominated Idlib province.
Britain-based war monitor, the Syrian Observatory for Human Rights, says some of the deadliest strikes of recent days have been carried out by government ally Russia.
But Moscow has denied launching any combat operations in the region since a ceasefire it agreed with rebel supporter Ankara went into effect earlier this month.
The Observatory said Russian bombardment targeted the west of Aleppo province on Friday without causing any casualties.
The latest wave of displacement compounds a dire humanitarian situation in the rebel-held northwest where more than 358,000 civilians had already been displaced by the intensified bombardment launched in December.
"This latest escalation has opened up a dangerous new front in the conflict," said Misty Buswell of the International Rescue Committee in a statement on Friday.
"Camps are full, health services are over-stretched, and the majority (of the displaced) are living either in flimsy, closely-packed tents... or crammed into communal shelters."
The Idlib region hosts at least three million people, many of whom have fled other parts of the country recaptured by the government and are in need of urgent humanitarian assistance.