Around 350,000 Syrians, mostly women and children, have fled a renewed Russian-backed offensive in the opposition-held Idlib province since early December, and have sought shelter in border areas near Turkey, the United Nations said on Thursday.
Conditions for people were deteriorating because of the increased hostilities, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said in its latest situation report.
Russian jets and Syrian artillery have pounded towns and villages in a renewed assault backed by pro-Iranian militias and aimed at clearing the opposition.
"This latest wave of displacement compounds an already dire humanitarian situation on the ground in Idlib," David Swanson, Amman-based UN regional spokesman for Syria, said.
Russian and Syrian jets resumed bombing of civilian areas in the opposition enclave two days after a ceasefire agreed between Turkey and Russia formally took effect on Sunday.
Russia's Defense Ministry on Thursday denied media reports that it had bombed civilian targets in Idlib province, saying there had been no military flights since a ceasefire was introduced on Jan. 9, the state-run RIA news agency reported.
Residents and rescuers said dozens of towns and villages were in ruins from weeks of intensive aerial bombing.
The Russian-led campaign has also knocked down dozens of hospitals and schools, international aid agencies said.
Karen AbuZayd, a UN war crimes investigator on Syria, told reporters in Geneva that many of the destroyed or closed schools in opposition-held areas were now being used as shelters for people fleeing the violence.
The latest wave of displaced people comes on top of close to 400,000 people who fled earlier fighting for the safety of camps near the Turkish border, UN officials said.
Paulo Pinheiro, chairman of the UN Commission of Inquiry on Syria, told reporters on Thursday that many uprooted families now in makeshift camps were running short of food and water.
The latest offensive has brought the Russian-steered military campaign closer to heavily populated parts of Idlib province, where nearly 3 million people are trapped, according to the United Nations.
Rescuers and residents said that on Thursday Russian and Syrian jets pounded the devastated city of Maarat al-Numan. It is one of the main urban centers in the province and lies on a main highway held by rebels.
The army and Iranian-backed militias are advancing towards the city. Its capture would be a strategic government gain in the current campaign, which is intended also to regain control of major trade arteries important to Syria's war-torn economy.
On Wednesday at least 21 civilians were killed in heavy aerial strikes, among them 19 people who died when bombs were dropped on a busy market place in the center of Idlib city, the provincial capital.