More than one million Syrians are trapped in besieged areas, a new report said in a challenge to the United Nations, which estimates just half that amount and has been accused by some aid groups of underplaying a crisis.

The fate of Syria's besieged is at the heart of peace talks that quickly fell apart last week in Geneva and are set to resume by Feb. 25. Negotiators for the opposition had insisted that the Syrian government stop besieging civilians before talks could truly begin.

The new Siege Watch report, issued yesterday by the Netherlands-based nonprofit PAX and the Washington-based Syria Institute, comes a month after images posted online of emaciated children and adults led to an international outcry and rare convoys of aid to a handful of Syrian communities.

The town featured in the images, Madaya, was not listed by the U.N. as a besieged community at the time. Aid workers who entered last month reported seeing skeletal people and parents who gave their children sleeping pills to calm their hunger.

The Siege Watch report said 1.09 million people are living in 46 besieged communities in Syria, far more than the 18 listed by the U.N. It says most are besieged by the Syrian government in the suburbs of Damascus, the capital, and Homs. In the eastern city of Deir el-Zour, about 200,000 people are besieged by both DAESH and the Syrian government. The report lists two communities besieged by armed opposition groups.

"Electricity and running water are usually cut off, and there is limited (if any) access to food, fuel, and medical care," the report said. Deaths have been reported from malnutrition, disease, hypothermia and poisoning while scavenging for food. Some communities have been besieged for months or years.

The estimates are based largely on information provided by local contacts in the communities, including local councils, medical workers and citizen journalists.

With the spotlight on the besieged, the United Nations last month raised its estimate by almost 100,000, saying that 486,700 people are affected.

That's still less than some aid groups and others estimate. They argue that the world body's numbers set the tone for humanitarian response efforts and that more urgency is needed. "Many remain unaware of the extent of the crisis, and the international response has been muted as a result," the Siege Watch report says.

The aid group Doctors Without Borders goes well beyond the figure in the Siege Watch report, estimating that 1.9 million Syrians live in besieged areas. Doctors Without Borders said it defines Syria's besieged areas as ones "that are surrounded by strategic barriers (military or non-military) that prevent the regular and safe inflow of humanitarian assistance and the regular and safe outflow of civilians, the wounded and the sick."

The United Nations places an estimated 4.5 million Syrians into a separate category called "hard to reach," a step below besieged. It defines that as "an area that is not regularly accessible to humanitarian actors for the purpose of sustained humanitarian programming as a result of denial of access."

Doctors Without Borders said it doesn't use that distinction, "as the medical consequences for both types of region are similar." Medical supplies are almost never allowed in, it said, and medical evacuations are rarely allowed out. The aid group has said that since convoys reached Madaya last month, at least 16 people there have died and at least 33 were in danger of dying of malnutrition.