Some 86 people have died in a year-long government siege of the Syrian town of Madaya, including 65 from starvation and malnutrition, two NGOs said on Tuesday.
The Syrian American Medical Society (SAMS) and Physicians for Human Rights (PHR) said the Syrian government's "stranglehold" on the town since July 2015 was responsible for the deaths.
Madaya, in Damascus province, grabbed international attention in late 2015 after reports its residents were starving to death because of food shortages.
Since then, four aid convoys have accessed the town, but the two groups said the deliveries had been insufficient.
"Last year, unspeakable images of Madaya's suffering emerged in the media, and we hoped that would trigger action to finally bring lifesaving aid into the town," said Elise Baker of PHR, a New York-based advocacy group.
"But UN humanitarian convoys that finally reached Madaya failed to provide the population with enough food, medicine, and medical equipment," Baker said.
"Dozens of Madaya's residents died because of these failures. And each day under siege brings the rest of Madaya's population one day closer to death."
An estimated 40,000 people remain in Madaya, surrounded by regime forces and allied fighters.
The United Nations says nearly 600,000 Syrians are living under sieges, mostly imposed by the government, though the tactic has also been used by rebel fighters and the Islamic State group.
The UN and aid groups have struggled to access those living under siege, and last month dozens of activist groups opposed to Syria's government accused the UN of "capitulating" to Damascus on the issue of aid delivery.
PHR and SAMS accused the Syrian government of removing items from humanitarian convoys to Madaya, including kits for treating malnourished children that were part of a February convoy.
"Subsequently, at least two children starved to death: an eight-year-old boy and a six-month-old girl," the groups said.
"Children are dying of starvation an hour from warehouses filled with food aid in Damascus," said Baker.
The groups urged an immediate end to sieges across Syria, and called on the UN Security Council to authorise humanitarian aid drops, air bridges, or airlifts, if necessary to help those under siege.
The UN has said it would formally request Syrian permission for humanitarian air drops if it was unable to reach areas via land convoys, but has yet to make such a request for Madaya.