Syrian government forces and rebels have begun implementing a deal to evacuate four besieged towns by carrying out a prisoner exchange.
State media said rebels had handed over 12 women and children and seven bodies. At the same time 19 rebels were freed.
It is the first stage of an agreement that should lead to thousands of people leaving rebel-held Madaya and Zabadani and pro-government Foah and Kefraya.
Residents of the first two towns said buses began arriving on Wednesday.
But a government co-ordinator for the deal told AFP news agency that rebels were "delaying" buses trying to reach Foah and Kefraya, and stressed the evacuations would have to be simultaneous.
The UN has described the situation in the four towns as "catastrophic", with more than 60,000 civilians "trapped in a cycle of daily violence and deprivation".
Many people are reported to have died as a result of shortages of food or medicine.
Foah and Kefraya, most of whose residents are Shia Muslims, have been encircled by rebels and al-Qaeda-linked Sunni jihadists since March 2015.
Madaya and Zabadani, which are predominantly Sunni, have been besieged since June 2015 by the Syrian army and fighters from Lebanon's Shia Islamist Hezbollah movement.
As part of what is known as the "Four Towns Agreement", the warring parties have allowed the UN and Red Cross to deliver aid on a few occasions in the past two years and to evacuate limited numbers of sick and injured people.
Those who want to be evacuated from Foah and Kefraya will reportedly be transported to government-controlled areas of Damascus, while Madaya and Zabadani residents will be taken to rebel-held areas around the city of Idlib and the town of Jarablus if they wish to leave.
The agreement also reportedly includes the entry of humanitarian assistance and a nine-month pause in fighting covering the four towns and southern parts of Damascus and its countryside.
Two weeks ago, UN Secretary General Antonio Guterres warned that any evacuation of civilians had to be "safe, voluntary, and to a place of their choosing".
"It is imperative that all those who are displaced through such agreements are allowed to return voluntarily, in safety and in dignity, to their homes as soon as the situation allows," a spokesman said.
A recent report by UN commission of inquiry concluded that last December's evacuation of people from parts of the northern city of Aleppo that were previously held by rebel forces had amounted to the "war crime of forced displacement".
Some 4.7 million people were living in hard-to-reach and besieged areas in Syria as of January, including 644,000 in UN-declared besieged locations.