The only medical doctor remaining in the besieged Syrian town of Zabadani, and a member of a rescue team, was shot dead by a sniper after treating a patient, according to Medecins Sans Frontieres.

In most war-torn areas of the country there are few doctors left, in some places none at all.

The doctor was named by the Associated Press agency as Mohammed Khous, a 70-year-old who was walking from a hospital to his son's house.

More than 250,000 people have been killed during the five-year conflict and the war has created the worst refugee crisis since World War II, and seen the rise of the Islamic State (IS) militant group.

Medecins Sans Frontieres regional co-ordinator Sam Taylor said Syrian authorities were removing life-saving equipment from aid convoys entering the besieged areas.

"This is a pretty dire situation because the number of medical staff in Zabadani is, well it's zero now," he said.

"But also in the wider diseases areas it's extremely difficult for medical staff to received the supplies they need to get the life saving treatment they do."

Mr Taylor claimed there were about 1.6 million people living in besieged towns in Syria and it was "extremely difficult" to provide food or medical supplies.

"Some aid convoys are being allowed into some of the besieged areas but there are checkpoints along the way and at those checkpoints, medical equipment, life-saving medical equipment and drugs are being removed by [Syrian] authorities," Mr Taylor said.

"It's almost impossible to get people out so medical evacuations are being actively prevented.

"So we're hearing about deaths in places like Madaya because people are not being able to be evacuated and get the treatment they need."