The evacuation of more than 3,000 Syrians from four areas as part of a population transfer has been postponed a day after a deadly blast killed more than 120 people, many of them government supporters.
The reasons for the delay were not immediately clear.
It came as shells fired by the so-called 'Islamic State' group on government-held parts of the eastern city of Deir el-Zour wounded two members of a Russian media delegation visiting the area, according to state-run Syrian news agency Sana.
Russia is a main backer of Syrian president Bashar Assad and Russian journalists enjoy wide access in government-held parts of the country.
Russia's Anna-News military news service, which employs the journalists, said one was wounded in the arm while the other suffered leg and stomach wounds.
The news service said the two were evacuated adding that their condition was "satisfactory".
The United Nations is not overseeing the transfer deal, which involves residents of the pro-government villages of Foua and Kfarya and the opposition-held towns of Madaya and Zabadani.
All four have been under siege for years, their fate linked through a series of reciprocal agreements that the UN says have hindered aid deliveries.
Rami Abdurrahman, who heads the Britain-based Syrian Observatory for Human Rights, and Hezbollah's Al-Manar TV, earlier said 3,000 people will be evacuated from Foua and Kfarya, while 200, the vast majority of them fighters, will be evacuated from Zabadani and Madaya.
Mr Abdurrahman and opposition activist Hussam Mahmoud, who is from Madaya, said the evacuation has been delayed.
Mr Abdurrahman said no permission was given for the evacuation to go ahead while Mr Mahmoud said it has been delayed for "logistical reasons".
It was not immediately clear if the evacuees feared attacks similar to Saturday's bombing.
Mr Abdurrahman said Saturday's blast, which hit an area where thousands of pro-government evacuees had been waiting for hours, killed 126.
He said the dead included 109 people from Foua and Kfarya, among them 80 children and 13 women.
No one has claimed the attack, but both the 'Islamic State group' and the al Qaida-affiliated Fatah al-Sham Front have targeted civilians in government areas in the past.
Anthony Lake, Unicef's executive director, said in a statement after six years of war and carnage in Syria "there comes a new horror that must break the heart of anyone who has one."
"We must draw from this not only anger, but renewed determination to reach all the innocent children throughout Syria with help and comfort," he said.
After the blast, some 60 buses carrying 2,200 people, including 400 opposition fighters, entered areas held by rebels in the northern province of Aleppo, Mr Abdurrahman said.
More than 50 buses and 20 ambulances carrying some 5,000 Foua and Kfarya residents entered the government-held city of Aleppo, Syrian state TV said, with some of them later reaching a shelter in the village of Jibreen to the south.
UN relief coordinator Stephen O'Brien said he was "horrified" by the deadly bombing, and while the UN was not involved in the transfer it was ready to "scale up our support to evacuees".
He called on all parties to uphold their obligations under international humanitarian and human rights law, and to "facilitate safe and unimpeded access for the UN and its partners to bring life-saving help to those in need".
Residents of Madaya and Zabadani, formerly summer resorts, joined the 2011 uprising against President Bashar Assad.
Both came under government siege in the ensuing civil war.
Residents of Foua and Kfraya, besieged by the rebels, have lived under a steady hail of rockets and mortars for years, but were supplied with food and medicine through military airdrops.
Critics say the string of evacuations, which could see some 30,000 people moved across battle lines over the next 60 days, amounts to forced displacement along political and sectarian lines.
In eastern Syria, an air strike by the US-led coalition on the village of Sukkarieh near the border with Iraq killed eight civilians who had earlier fled violence in the northern province of Aleppo, according to Deir Ezzor 24, an activist collective, and Sound and Picture Organisation, which documents IS violations.
Air strikes by the US-led coalition had killed dozens of civilians over the past several weeks as the battle against the extremists intensifies in Syria and Iraq.