A joint project collecting data from a network extending deep into trapped Syrian communities came to the grim conclusion that inadequate UN reporting may also "inadvertently encourage the expansion of the Syrian government's 'surrender or starve' strategy."
Citing the example of the town of Madaya where 46 people have died from starvation since December, the report warned the true extent of the crisis was being masked by what it said was the UN's "under-reporting" of the situation on the ground.
Pictures of severely emaciated adults and children in Madaya emerged late last year, and some humanitarian access was finally allowed into the town after a global outcry.
But Madaya did not even figure on a UN list of trapped communities, said the report from Siege Watch, a joint project launched by the Washington-based Syria Institute and PAX, a peace organisation based in the Netherlands.
New data gathered by the NGOs listed 46 communities around Syria where it said a total of 1,099,475 people are besieged.
The overwhelming majority are encircled by forces loyal to Syrian President Bashar al-Assad.
The figure could be even higher as the reporting began in November 2015, before the start of the latest regime assault to encircle rebel-held areas of northern Aleppo, which has sent tens of thousands fleeing.
People in besieged Syrian communities had an "elevated risk of death" due to deprivation from lack of food as well as electricity and running water, the report said.
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"The scale of the crisis of besieged areas in Syria is far worse than the UN Office for the Coordination of Humanitarian Affairs (OCHA) has acknowledged," the report said.
In response, Syrians "have begun to acclimatise to a more primitive lifestyle," adopting "survival tactics" such as rooftop gardening and burning plastics to extract oil.
Syria's nearly five-year conflict has claimed 260,000 lives and displaced half the population since March 2011.
The latest UN figures released in January put the number of people under siege at 486,700 people, with more than living half in areas encircled by regime forces.
But Siege Watch said its data pointed to "continued under-reporting of the siege crisis in Syria" in monthly UN reports.
The UN's "characterisation does not accurately reflect the situation on the ground," the report said.
Out of the 46 besieged communities, only two -- the towns of Fuaa and Kefraya in Idlib province -- were being held hostage by opposition forces.
A third area -- Deir Ezzor city where some 200,000 are trapped -- was encircled by both jihadists from the Islamic State group and regime forces, it said.
The report said the UN's misreporting of the numbers under siege meant that many people remained "unaware of the extent of the crisis and the international response has been muted as a result."
Peace talks aimed at trying to end the conflict were suspended until February 25, with the UN special envoy Staffan de Mistura saying "more work" was needed to prepare all sides for the negotiations.