The Syrian government and rebel groups have begun an operation to move people away from four besieged towns, activists say.
People from the north-western towns of Foah and Kefraya are being taken to government-held areas near Aleppo.
Evacuees from rebel-held Madaya, near Damascus, have been bussed to Idlib province.
It is not clear if nearby Zabadani, included in the deal, is also being evacuated.
More than 30,000 people will be moved.
Last month, the UN described the situation in the four towns as "catastrophic", with more than 64,000 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 meanwhile 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 the Red Cross to deliver aid on a few occasions in the past two years and to remove 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 Aleppo, 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 evacuation deal has been brokered by Iran, an ally of Syria's President Bashar al-Assad, and Qatar, which supports the rebels. But critics say it amounts to forced demographic change.
Some 4.7 million people live in hard-to-reach and besieged areas in Syria, including 644,000 in UN-declared besieged locations.
Meanwhile, the foreign ministers of Russia, Syria and Iran are due to meet in Moscow in the first meeting of the three allies since the US fired 59 cruise missiles at a Syrian airbase near Homs.
The US says its strike was in response to an alleged Syrian government chemical attack in the rebel-held town of Khan Sheikhoun, in which more than 80 people were killed.
Mr Assad has denied carrying out the attack, calling the reports "fabricated".
The ministers meeting in Moscow are expected to consider their next move, including what sort of investigation they might back, the BBC's Ben James in Beirut reports.
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