Warnings of widespread starvation in Syria are growing as pro-government forces besiege an opposition-held town and winter bites, darkening the already bleak outlook for peace talks the United Nations hopes to convene this month.

The blockade of Madaya, near the Lebanon border, 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.

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The United Nations says it has received credible reports of people dying from starvation in the town. The Syrian Observatory for Human Rights, which tracks the war, puts the toll at 10, while opposition activists say it is in the dozens.

Reuters could not independently confirm the reports.

“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.”

The United Nations said the Syrian government on Thursday approved access and it was preparing an aid delivery “in the coming days”, along with a delivery to two villages in northwest Syria besieged by rebels.

Residents in Madaya, where the United Nations says some 40,000 people are at risk, meanwhile make do with water flavoured, where available, with spices, lemon, salt and vinegar, said Abu Hassan Mousa, the head of an opposition council in Madaya.

Where rice or powdered milk are available, the prices can reach some $US300 ($A425) a kilo, residents said.

With half a metre of snowfall this week, furniture, doors and wooden fixtures and fittings are being burnt to heat homes, said Ali, an opposition activist.

“Negotiations have no meaning all the time we are besieged, all the time we are hoping for a cup of milk for a child. What are we going to negotiate over? Our dead?” he said.