(AP) -- Nearly 650,000 Syrians are living in besieged communities in the country's civil war, more than three times the UN estimate, according to a new report that gives a graphic account of hundreds of deaths in areas the world has struggled for years to reach. The report says Syria's government is responsible for the siege tactics that have led to deaths by starvation, dehydration and the lack of medical care. The document does not look at what it calls the short-term siege tactics used by Islamic State, which has beheaded and massacred its opponents in the area straddling the Syria-Iraq border currently under its control. The "Slow Death" report, obtained in advance by Associated Press, is by the Syrian American Medical Society, which supports medical workers in besieged areas. The organisation presented its findings on Thursday to UN officials and to a closed-door meeting sponsored by the United States, Britain, France and other states and organised by Qatar. The UN estimates that 212,000 Syrians live in besieged areas beyond the reach of humanitarian aid. But the new report, to be released next week, says the UN is too narrowly defining "besieged" and is inadvertently underplaying the crisis. It says more than 640,200 people are besieged. It also echoes claims by an increasing number of aid groups that the international response to the overall conflict, particularly by the deeply divided UN Security Council, has failed. "We're not talking about quote-unquote terrorists, we're talking about families who have nothing to do with armed groups," the president of the Syrian American Medical Society, Dr Zaher Sahloul, told AP. The group describes itself as being a neutral medical organisation. More than 220,000 people have been killed in Syria's civil war, which began with protests against President Bashar Assad. The government has been repeatedly accused of using siege tactics against its own citizens. The new report says those tactics have had a devastating impact on dozens of communities trapped in the Syrian conflict, which this month enters its fifth year. It identifies 38 communities that it says should be considered besieged beyond the 11 areas that the United Nations recognises. The report's website, Syriaundersiege.org, lists 560 people who have died in besieged areas, and has posted photos of 345 of them. To emphasize the civilian impact of the deaths, the organisation says it did its best to keep out people with links to armed groups. The report says all the 560 deaths in that dataset "were in areas besieged by the Syrian government". Syria's government has denied using siege tactics. "If weapons and instruments of death are reaching those areas ... how can they be said to be besieged?" Syria's UN ambassador, Bashar Ja'afari, wrote to the UN secretary-general in late February, asserting that the weapons went to "armed terrorist groups". Sahloul and others on Thursday presented diplomats, including US Ambassador Michele Sison, with some of the images of the dead that accompany the report. The website's photos of victims include emaciated children. "A lot of people cried, it was kind of intense," said Valerie Szybala, the author of the report, who attended the meeting. She said that Russia and China, who have vetoed resolutions attempting to take action on the Syrian crisis, did not attend. Szybala said the medical organisation knows well the politics that have brought the council to a near-standstill on Syria. "But we're trying to find creative ways to address this, because it's not acceptable to just sit around. People are dying," she said. The report argues that the UN Office for the Coordination of Humanitarian Affairs (OCHA) should immediately reconsider the way it designates besieged areas and not be too quick to remove a community from its existing list, especially when the only access to a community is controlled by the "besieging party". It also calls on the UN to arrange for international monitors to make sure all parties uphold the terms of local ceasefires, which at times are a trigger to remove a community from the list of besieged. UN OCHA officials in their meeting on Thursday said they had no issues with the new report's methodology or numbers and that they have had to rely on third-party estimates because they don't have people on the ground in besieged areas, Sahloul and Szybala said. OCHA officials had no immediate comment on the new report. Other international aid organisations have said that it is extremely difficult to get information from Syria's besieged areas, just as it's hard to get aid in. "My fear is to go for a battle of figures when the most important point is that there are a lot of people starving," said Doctors Without Borders emergency coordinator Pierre Boulet Desbareau. "Two hundred thousand or 600,000, the issue is the same."