A steep increase in violence in Syria left 12 civilians dead in the last major bastion of opposition forces as aid groups said the country faced a humanitarian catastrophe if the flow of cross-border supplies stopped reaching the region.
The UN said bombings by the Syrian regime and Russian forces in the north-west province of Idlib since December 16 forced tens of thousands of people to flee their homes.
The world body has called for “immediate de-escalation” and warned there would be further mass displacement if the violence continued.
The Idlib region is home to about three million people, including many displaced by years of violence in other parts of Syria.
Damascus vowed to take back the area from militant control and bombardment has continued despite a ceasefire announced in August.
Exacerbating an already dire humanitarian situation, Russia and China on Friday vetoed a UN Security Council resolution that would have extended cross-border aid deliveries to four million Syrians for a year.
The move, criticised by member states such as the US, raised fears that UN-funded assistance could stop entering opposition-held parts of Syria from January unless an alternative agreement was found.
US Secretary of State Mike Pompeo on Saturday described the Russian and Chinese veto as shameful.
"To Russia and China, who have chosen to make a political statement by opposing this resolution, you have blood on your hands," he said.
Aid group Oxfam said people in need, many of them displaced several times, relied on the deliveries.
"There is no realistic way of reaching hundreds of thousands of these families" except though cross-border operations, it said.
Abu Zakour, 70, lives in a camp in northern Idlib and said he feared for the displaced if the deliveries were halted.
"Had it not been for the aid, we would have died from hunger," he said.
The fears of an aid crisis came as violence intensified in Idlib.
On Saturday, air strikes by the regime and its ally Russia killed 12 civilians and injured 36 others, the Syrian Observatory for Human Rights said.
That came on the third day of clashes between regime loyalists and Idlib militants, with about 140 people killed on both sides, the Observatory said.
Battles since Thursday have killed 67 extremists and 15 allied rebels, it said.
The fighting also left 57 regime loyalists dead, including at least seven who were killed on Saturday in a car bombing by the country's former Al Qaeda affiliate, Hayat Tahrir Al Sham, the Observatory said.
The regime, which controls 70 per cent of Syria, launched an offensive against militants in Idlib in April, killing about 1,000 civilians and displacing more than 400,000.
Since August, the area has supposedly been protected by a ceasefire announced by Moscow, but bombardment has continued.
A sharp increase in violence this month highlighted the continued need for humanitarian aid, which currently flows into Syria through UN-designated checkpoints without formal permission from Damascus.
Four million Syrians directly benefit from the deliveries, with 11 million receiving international aid inside the country eight years into its civil war.
Last Friday, International Rescue Committee chief David Miliband warned of the effects of scaling back aid.
"With a fresh spate of attacks in Idlib, and continued brazen flouting of international humanitarian law, cutting humanitarian aid is the last straw," he said.
Losing aid could not come at a worse time of year, as heavy winter rain floods camps for the displaced.
UN-supported aid organisations in north-west Syria say they may have to halt operations if cross-border support stops.
"This will paralyse the humanitarian effort in Syria's north," Maamoun Kharbout of aid group the Violet Organisation said.
Meanwhile, suspected drone attacks targeted three government-run oil and gas facilities in central Syria at dawn on Saturday, the oil ministry said.
There was no immediate claim of responsibility.