BEIRUT—The second stage of a troubled population transfer in Syria stalled on Thursday as rebel and government negotiators argued over the identities of the prisoners to be released as part of the exchange.

More than 3,000 people found themselves trapped at the handover point between the two sides on the outskirts of Aleppo city, where a car bomb on Saturday killed more than 130 people, half of them children.

That blast, which remains unclaimed, rushed the two sides to complete the first stage of the transfers within the next day, but injected an element of terror into the chain of operations, which are expected to last for 60 days and see up to 30,000 Syrians moved across battle lines.

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Critics of the arrangement, which was brokered by Qatar and Iran and involves four besieged areas, have decried it as a forcible transfer that is altering the country’s demographics along political and sectarian lines.

At least 112 people have died after a bomb hit buses carrying evacuees on Saturday from besieged Syrian towns, according to the Syrian Observatory for Human Rights.

“Our mental state is very bad. There are fears that the deal is going to stall,” said Amer Burhan, a medical worker from Zabadani, who is trapped at the exchange point.

Some 3,000 residents of two pro-government villages, Foua and Kfarya, left Wednesday in 45 buses bound for government-controlled Aleppo. Another 11 buses carrying some 500 people, including opposition fighters, left Madaya and Zabadani, near Damascus, heading toward the northern rebel-held Idlib province.

The Britain-based Syrian Observatory for Human Rights monitoring group also reported the transfer, which it says includes 800 armed men from both sides. Some 160, mostly gunmen, had remained in Zabadani.

But the exchange has been delayed over the identities of the 750 prisoners who are supposed to be released as part of the deal, according to Yasser Abdelatif, a spokesman of one of the rebel factions involved in the exchange.

Rebels grew concerned the government was not planning to release political prisoners, after authorities provided them with a list of some of the names, said Fahd al-Musa, who heads the activist-run Syrian Commission for Releasing Detainees, which the rebels consulted to check the names.

“They should release people like Rania Abbasi,” said al-Musa, referring to the Syrian dentist who activists say was taken with her husband and six children from their Damascus home in 2013. They have not been heard from since.

The U.N.’s chief humanitarian adviser for Syria meanwhile said aid agencies have not been able to reach as many besieged Syrians this year as they did over the same period last year.

Jan Egeland told reporters in Geneva that the front lines have shifted but civilian suffering has “remained the same” in 2017.

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Of nearly 5 million Syrians living in besieged or hard-to-reach areas, agencies have only been able to reach 564,000 this year to date, he said.

He drew particular attention to the dire humanitarian situation in the opposition-held Eastern Ghouta suburbs of the capital, Damascus.

About 400,000 Syrians are trapped inside the area, which is besieged by government forces. Aid agencies have been unable to reach Douma, the largest town inside the besieged area, in half a year.

Shortages of food and other basic goods have pushed prices “through the roof,” said Egeland. “We get reports that there is no bread or wheat on the market at all.”

Siege has been a pillar of the government’s military strategy against its domestic opponents throughout the six-year-old civil war.

By tightly restricting the delivery of food and medical relief, the government has secured the surrender of broad swathes of the opposition-held countryside around Damascus, as well as the former opposition enclaves in the cities of Aleppo and Homs.

Syrians wait by buses on the outskirts of Aleppo as an evacuation deal stalls.

The Syrian government and rebels are allowing up to 30,000 people to leave besieged areas over the coming two months as part of the mutually agreed population transfer. The last residents of the formerly rebel-held town of Zabadani, outside Damascus, were evacuated Wednesday.

Egeland said the sieges should be lifted, and that no Syrians should be forced from their homes through starvation. The U.N. is not taking part in the transfer.

The U.N.’s Special Envoy for Syria, Staffan de Mistura, said at the same press conference that it was his aim to resume Geneva peace talks in May, but “we are watching very carefully the developments on the ground.”

The international body has held five rounds of talks since 2012.

De Mistura acknowledged that a ceasefire brokered by Russia and Turkey at the end of last year “doesn’t seem to be working.” He said it would be addressed at upcoming meetings between the various parties in Kazakhstan.

The U.S.-led coalition against the Islamic State group meanwhile said it is looking into reports that an April 17 airstrike on the Syrian town of Boukamal, on the border with Iraq, killed several civilians. It said it could not yet confirm the allegations made by the Islamic State group, which holds the town, and by other monitoring groups.

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