A group of Syrian doctors have signed an open letter to David Cameron calling on the RAF to carry out air drops of aid to starving civilians still living in siege conditions despite a ceasefire in the country's five-year war.

The doctors say that civilians are suffering from “mass starvation”. Currently 18 areas are under siege, nearly all by the Assad regime’s forces, despite a UN resolution demanding access be granted to aid convoys.

Failure to deliver the aid has been cited by opposition spokesmen in Geneva as one reason for suspending participation in peace talks sponsored by the United Nations.

The doctors are supporting a campaign being waged already by a number of MPs from both major parties for the RAF to use its expertise in high-altitude air drops to deliver food and medicine directly.

“Air drops are not a perfect solution, but they are the best hope our people have of avoiding starvation,” they write.

“It is incomprehensible to us that world leaders believe they can reach a peace agreement in Geneva while Syrians are being starved to death in Daraya.”

The opposition agreed to peace talks only after receiving a guarantee that aid would be allowed through.

Since then, UN officials have accused the Assad regime of holding up deliveries by refusing to reply to requests for permits to travel through army lines, and even of removing medical aid from trucks.