The Syrian government has been accused of putting up bureaucratic hurdles for aid organisations, leaving its civilians in desperate need of medicine and food.
The authorities are also blamed for systematically stripping convoys of their most vital supplies: medicine and medical equipment.
Some aid organisations say the red tape is causing deaths.
"Physicians for Human Rights has not documented specific numbers but reports we are seeing from other organisations show dozens have died in Madaya alone, let alone the other dozens of besieged areas," said Elise Baker, a representative of Physicians for Human Rights.
The UN Security Council has passed a total of six resolutions demanding that humanitarian deliveries be allowed to all areas of Syria, but it has no way of punishing the government for the hurdles.
The Syrian conflict has left more than 250,000 people dead in the last five years.
A tenuous ceasefire, brokered by Russia and the US, has been in place since February but Syria has continued to bomb rebel-controlled areas in Aleppo.
Nearly 300 people have been killed in the recent upsurge of violence.
