The besieged Syrian town of Madaya is facing starvation again because meagre supplies delivered last month are already running out, residents have said, as the United Nations was accused of severely underestimating the number of people suffering under blockades around the country.

More than a million Syrian civilians are living in besieged towns and villages, according to the aid organisation Siege Watch, more than double the number listed in UN data. That number could rise sharply if the government troops advancing on Aleppo cut off the city’s last supply line, with the UN warning on Tuesday that up to 300,000 civilians could be stranded in the city that was once Syria’s biggest urban centre.

“If the government of Syria and allies sever the last remaining flight route out of eastern Aleppo city, it would leave up to 300,000 people still residing in the city cut off from humanitarian aid unless cross-line access could be negotiated,” the UN said in an emergency bulletin on the situation.

Aleppo siege looms as pro-Assad forces cut opposition supply lines Read more

A major aid group working in the area told the Guardian that an even greater number of people – about 400,000 – were at risk. The UN warned that up to 150,000 refugees may also flee from Aleppo towards the border with Turkey, joining tens of thousands already living there in squalid camps.

The use of “starve or surrender” tactics by the forces of President Bashar al-Assad came under increased scrutiny after photos and videos emerged last month of the emaciated, desperate children of Madaya, where dozens of people are thought to have starved to death before an aid convoy was allowed through.

Now those limited supplies are almost gone. “Today I ate grass, to make the aid last longer,” said a young teacher and activist in Madaya, who was one of the first to alert the world to the extreme deprivation in the former resort town. “Some families have already run out. In 10 days most of us will have nothing to eat.”

The January shipment came too late to help some of the weakest people, and now the town is bracing for more losses. It is surrounded by minefields and snipers who are said to have killed several of the most desperate inhabitants as they tried to slip away.

“Since the aid entered Madaya, 16 men and women have died, and four children,” said the teacher, who asked not to be named for fear of reprisals against family members in government-controlled areas.

The suffering is being mirrored across Syria, Siege Watch warned, saying the UN was failing to register more than half of the blockades and citing Madaya as one of the forgotten sieges.

“The scale of the crisis of besieged areas in Syria is far worse than the UN … has acknowledged,” said the report by Siege Watch, an initiative organised by the Dutch charity Pax and the Syria Institute, a US non-profit research group.

“New data gathered by Siege Watch shows that there are well over a million Syrians under siege … The scope and severity of sieges across Syria continues to grow.” The report added that the deliberate starvation of civilians was a war crime.

The majority of people were under siege from government forces, with many also blockaded by Islamic State (Isis) fighters, the report said. In the city of Deir ez-Zor, Isis has more than 200,000 people under siege, but government forces are complicit in the suffering because they control the city’s airport and have refused to allow aid flights to bring supplies for civilians, Siege Watch said.

“While the UN reporting describes Deir ez-Zor as being besieged by Isis, residents living there feel that they are besieged by both Isis and the Syrian military,” the group said. “The Syrian government’s actions towards the besieged neighbourhoods support this claim.”

Madaya was not listed as under a blockade in a year-end UN report on the situation in Syria, even though UN officials had known about the extent of suffering there for months. The UN now considers Madaya besieged and helped to organise the aid convoy in January.

The Damascus suburb of Mouadamiya, where Siege Watch estimates that about 44,000 people are under siege, did not appear on the UN list in December either. A residents’ committee earlier this month demanded UN help and accused it of “ignoring the suffering of the civilians ... until [they] are at the brink of death”. The International Committee of the Red Cross considers the suburb besieged and delivered aid for thousands of people in recent days, Reuters reported.

Not all the sieges are as severe as the blockade on Madaya. The Siege Watch report designates three tiers of intensity, as well as areas on a watch list of potential siege situations. It warns that the UN is overlooking some of the looser blockades, where black market traders and those with government links can smuggle food in to sell at “extortive and predatory prices”. These systems drain money from the area and ultimately lead to starvation, the report said.

“While these practices may prolong the decline of the humanitarian situation in the besieged areas, they do not change the end result,” the report said. The group says its information comes from contacts on the ground. It plans to publish an estimate of the siege situation every three months.

Madaya: residents of besieged Syrian town say they are being starved to death Read more

Speaking on the sidelines of a government summit in Dubai, the UN’s deputy secretary general, Jan Eliasson, acknowledged that lack of progress on breaking the sieges had contributed to the suspension of peace talks in Geneva. “You have starvation and sieges … We have a lack of access to up to 400,000 people and we need to have movement on those areas very soon,” he told the Guardian.

“In my negotiations I’ve expected the level of violence to go down when the talks start. In this case, we had an escalation, and this is one of the reasons why we took a pause. We expected progress on the sieges and the humanitarian access to that big group of people, and nothing came out of that.”

The peace talks coincided with an escalation of violence in Syria and an intense aerial campaign by Russia, which intervened on the side of Assad. Eliasson said he had proposed lifting the sieges as a confidence-building measure ahead of the talks but had only managed to achieve “limited progress”.

He reacted angrily to accusations that the UN was not doing enough to end starvation sieges in Syria. “We are fighting like hell,” he said. “We lost a hundred people there, so when I hear this, I lose my cool. Because we are doing everything. [UN special envoy] Staffan [de Mistura] is working day and night. I am giving instructions; I am on it every day.

“Don’t shoot the messenger; we have no army, we depend on others, we are as strong as member states want us to be, and we pay the price all the time,” he said.