The UN said there were 40,000 people -- half of them children -- who needed immediate lifesaving assistance in Madaya, where access has been restricted by pro-regime forces.
Damascus on Thursday gave permission for UN agencies to send relief to the town, following reports of starvation deaths among civilians, many of whom have been displaced from the neighbouring rebel stronghold of Zabadani.
Doctors Without Borders, known by its French acronym (MSF), said that of the 23 people who died of starvation, six were less than a year old, and five were above 60.
The deaths occurred at the local MSF-supported health centre, the charity said.
Another 13 people who tried to escape in search of food have been killed when they stepped on landmines laid by regime forces or were shot by snipers, according to the Syrian Observatory for Human Rights, a British-based monitoring group.
Signup to our newsletter and follow us on Facebook and Twitter!
"This is a clear example of the consequences of using siege as a military strategy," MSF's operations director Brice de le Vingne said in a statement.
Medics had been forced to feed children with medical syrups as the only available source of sugar and energy, he said, describing Madaya as "effectively an open air prison" for nearly half of its residents.
"There is no way in or out, leaving the people to die."
MSF welcomed the decision from Damascus to allow food supplies, but stressed that "an immediate life-saving delivery of medicine across the siege line should also be a priority."
In Geneva, UN agencies said the aid convoy would head to Madaya in the coming days, although the specifics were still being finalised.
"The situation is ghastly," said UN rights office spokesman Rupert Colville, indicating that details of the casualties and the extent of the suffering in Madaya were difficult to verify given the limited access.
Despite numerous UN requests, Madaya last received humanitarian assistance in October.