Aid groups hope to deliver more food and medicine on Thursday to three besieged Syrian towns, including Madaya, where residents have reportedly starved to death, a UN official said.

Linda Tom, a spokeswoman with the UN's Office for the Coordination of Humanitarian Affairs (OCHA), confirmed to AFP that new aid deliveries were planned for Madaya, where more than two dozen people are thought to have died from lack of food, and two other towns.

"We are planning operations to Madaya, Fuaa and Kafraya on Thursday, which will be followed by a third delivery in the following days," she said on Wednesday.

She added that assistance for a fourth town, Zabadani, was being planned for a later date.