The only remaining doctor in the besieged Syrian town of Zabadani has been killed by sniper fire, days before the UN said it would begin a major medical evacuation from areas under siege.

The doctor and a member of a rescue team were shot last week after treating a patient, Médecins Sans Frontières (MSF) said. A doctor in the neighbouring town of Madaya named the man as Mohammed al-Kos, a surgeon in his 70s.

Both towns have been under siege by the Assad regime and Hezbollah forces since July last year, in retaliation for the use of the tactic on two towns in northern Syria by rebel forces. Reports have emerged from the towns of people starving to death and being forced to eat grass and stray cats.

Despite a ceasefire which came into force on 27 February, the UN and aid agencies say getting aid into besieged towns has been proved difficult. MSF says aid convoys are also restricted from transporting some life-saving medical supplies.

Some areas, including Daraya and Douma, remain completely cut off from aid. MSF estimates there are up to 1.9 million Syrians currently under siege. The UN announced it hoped to begin the evacuation of up to 500 sick and wounded people from Madaya, Zabadani and the two rebel besieged towns of Fua and Kefraya within days. But it will be too late for many, including five people in Madaya who died in the town in recent days.

Among them were three boys who died of their wounds after they were caught in a landmine explosion, and a young man who starved to death, after Hezbollah ignored pleas for them to be evacuated.

Jan Egeland, the co-chair of the task force on humanitarian access in Syria, said the deaths were “totally unnecessary.”

There are hardly any doctors left in Syria’s besieged areas. In Madaya, there are five medics left, including two dentists and one vet. Dr Mohammed Darwish, one of the town’s dentists, said he has had to deal with all levels of medical care, from first aid to surgery.

“This is simply our situation,” he said. “It has become habit.”