The UN says it is aiming to begin a large-scale evacuation of wounded and sick people from four besieged towns in Syria within the next week.

Jan Egeland, who heads a UN-backed humanitarian taskforce for Syria, said a big medical evacuation was planned for Madaya and Zabadani, two towns near Damascus, blockaded by the government, and Fuaa and Kafraya, which are besieged by rebels in Syria's northwest. He said that in all, up to 500 people could be evacuated.

The UN began scaling up deliveries of aid in Syria after a ceasefire went into force in February. But although a new round of peace talks is scheduled to begin next week, Egeland said access was slowing down.