BEIRUT, Lebanon — The process of evacuating residents from the last rebel-held section of Aleppo broke down on Friday, a day after Syrian government buses and ambulances began taking residents out of the besieged area.
The reason for halting the process after more than 8,000 people, mostly civilians but also some insurgent fighters, had been taken to safety was not immediately clear. Nor was it known whether the operation had been suspended temporarily or permanently.
Syrian state news media accused the rebels of seeking to smuggle weapons out of the area and of firing on the convoys of evacuees, while anti-government activists said pro-government militias had blocked the passage to protest the continued siege of two Shiite villages by rebels.
The United Nations said it was asked to leave the area where residents of eastern Aleppo had gathered to be evacuated, but that it did not know why the process had been halted.
“No reason was given,” Elizabeth Hoff, the World Health Organization’s representative in Syria, told journalists in Geneva, speaking by telephone from the United Nations base in west Aleppo.
“A very high number of people” still need to get out of eastern Aleppo, including many women and children under 5, Ms. Hoff said. It is unclear how many people remain in the rebel enclave, but most estimates put the figure at 50,000 to 100,000.
Many people had gathered at evacuation points Friday morning, only to return to their neighborhoods when the operation stopped.
Ms. Hoff said she assumed the decision came from officials in Russia, whose military is overseeing the evacuation. The United Nations had no contact with Syrian officials throughout the operation on Thursday.
Hospitals in western Aleppo were “overwhelmed” by residents of nearby government-held areas, as well as by people from the east who were injured in the conflict or needed treatment for chronic diseases, she said.
The evacuations, brokered by Turkey and Russia and carried out by the Syrian Arab Red Crescent and the Red Cross, began Thursday morning after violence in the war-torn city caused delays. They continued overnight, providing hope that the cease-fire between the government and rebel forces in the city would hold.
Before Friday’s suspension, more than 8,000 people had been removed from the rebel enclave, according to the state news media and the Syrian Observatory for Human Rights, which opposes the Syrian government and tracks the conflict from Britain.
The Syrian Observatory attributed the suspension to the failure of a separate operation to evacuate civilians from two Shiite villages in nearby Idlib Province, Fua and Kfraya, that have long been surrounded by rebel forces.
While buses had departed Thursday for the villages, no residents had left by Friday morning, leading gunmen from pro-government militias to block the road out of the rebel enclave in Aleppo.
Robert Mardini, the Middle East director for the International Committee of the Red Cross, confirmed the suspension but did not provide the reason.
The two Shiite villages, which have been surrounded for years by Sunni rebels forces that often shell them, were not originally part of the evacuation deal. They were added after pro-government gunmen fired on the evacuation convoys on Wednesday.
The evacuation deal followed a long siege of the rebel-held area by Syrian government forces and their allies, during which shelling and airstrikes by Russian and Syrian jets killed hundreds of people and reduced much of the area to rubble. Rebels also shelled government-held areas of the city, killing and wounding civilians.
President Bashar al-Assad, speaking in a video released by his office on Thursday, described the evacuation as “history that is being written,” comparing it to historic events like the birth of Christ and the world wars.
Secretary of State John Kerry denounced the Syrian government in harsh terms. “The Assad regime is actually carrying out nothing short of a massacre,” he said.
Under the terms of the deal, civilian evacuees can stay in government-held areas or can go to rebel-held areas to the north or west. Evacuated fighters can go only to rebel areas.
Turkey says it is planning to set up camps for the displaced in northern Syria, near their border.
President Vladimir V. Putin of Russia said on Friday that he was working with Turkey to begin a new round of talks in Kazakhstan’s capital, Astana, aimed at ending Syria’s nearly six-year war.
Previous peace talks, overseen by the United Nations and held in Geneva, have failed to end the war, which has killed more than 500,000 people and displaced about half of Syria’s population.
The crisis in eastern Aleppo has divided world opinion. Many Western countries, Turkey and Persian Gulf states have backed the rebels, criticizing the Syrian government and its allies for their prosecution of the war.
For their part, Syria, Russia and Iran have characterized the rebels as “terrorists” serving foreign agendas and seeking to destroy the country. A report on Friday from the Syrian state news agency, Sana, said that more than 8,000 “terrorists and their families” had been evacuated from Aleppo.
The United Nations and the Red Cross say that most of those leaving are civilians.