The United Nations has said the Syrian government has allowed access to an opposition town near the border with Lebanon where concerns about widespread starvation are growing.

The UN said in a statement it was preparing to deliver humanitarian assistance in the coming days to the town of Madaya, which is being besieged by pro-government forces since July. 

The World Food Programme said the lives of up to 40,000 people are at risk in Madaya, which last received an aid delivery in October.

"We were living on tree leaves, on plants, but now we are struggling in a snow storm and there are no more plants or leaves," said Majed Ali, 28, an opposition activist who spoke to Reuters by phone from Madaya.

"I was 114 kilos before the siege. Now I am 80." 

Madaya residents make do with water flavoured, where available, with spices, lemon, salt and vinegar, another resident said. 

Blockades have been a common feature of the nearly five-year-old civil war in Syria that has killed an estimated 250,000 people.

The blockade of Madaya has become a focal issue for Syrian opposition leaders who told a UN envoy this week they will not take part in talks with the government until it and other sieges are lifted.

Aid agency Médecins Sans Frontières (MSF) said 23 patients in the health centre supported by the charity had died of starvation since 1 December.

It called for sick patients to be evacuated for treatment in safe places. 

"The medics we support report injuries and death by bullet and landmine wounds from people that tried to leave Madaya," said Brice de le Vingne, MSF Director of Operations.

"The desperation is getting so acute that yesterday there were scenes of rioting as people tried to seize the last remaining food available at the MSF-supported food-distribution point, intended to provide for the most vulnerable.”