A UN aid convoy is expected to leave for Madaya, a Syrian town under government siege, where people are reported to have starved to death.
Emergency food supplies had been due to be sent to the rebel-held town on Sunday but the operation was delayed.
There are about 40,000 people in Madaya, near Lebanon’s border, with residents said to be eating pets and grass to survive.
A similar aid operation is expected for two northern towns besieged by rebels.
Meanwhile, Brice de la Vigne from the Medecins Sans Frontieres (MSF) medical charity described the situation in Medaya as “quite horrific”.
Mr de la Vigne, whose organisation has been in contact with doctors inside Madaya, told the BBC that more than 250 people there had “acute malnutrition”.
He added that “10 of them need immediate medical evacuation” or they would die.
The World Food Programme (WFP) earlier had hoped to take a first shipment of food and medicine to Madaya.
It was not clear what caused the delay but the BBC’s Jim Muir in Beirut says negotiating access across battlefronts in a siege situation has always been a tricky business.
It involves agreement at the top political level on both sides of the conflict, as well as individual fighters on the ground.
Emergency deliveries are also planned for two government-held villages in the north – Kefraya and Foah.
Blockades have been a feature of Syria’s civil war but the plight of Madaya has drawn international attention, partly due to images emerging of severely malnourished residents.
Up to 4.5 million people in Syria live in hard-to-reach areas, including nearly 400,000 people in 15 besieged locations who do not have access to life-saving aid.
Madaya has been besieged since early July by government forces and their allies in Lebanon’s Shia Islamist Hezbollah movement.