Amateur video released on a social media website appears to show residents in the Syrian town of Madaya being handed packets of salt. Warnings of widespread starvation are growing as pro-government forces besiege the 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 border with Lebanon, has become a focal point 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.

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. The UN said the Syrian government approved access on 7 January 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 $300 (£206) a kilo, residents said.

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