The Syrian government suspended the evacuation of civilians and fighters from the last rebel-held parts of Aleppo on Friday, leaving thousands of people trapped and uncertain of their fate.
Appeals came in from around the world for the evacuations to resume, with UN Secretary General Ban Ki-moon warning that Aleppo had become "a synonym for hell".
The United Nations urges "the parties to take all necessary measures to allow safe resumption of this evacuation process", he said in New York.
Turkish President Recep Tayyip Erdogan also called for all sides to respect the Aleppo deal.
Rebel backer Ankara, which helped broker the accord with regime ally Moscow, said thousands of people were still waiting to leave.
Turkey and a Syrian military source said the evacuation had been suspended but was not yet over, while Moscow insisted the operation was now "complete", with all women and children moved from the city.
Amid the confusion, a convoy of evacuees that had already left the east when the operation was suspended was forced to turn back, an AFP correspondent said.
The delicate operation to bring the last civilians and rebels out of east Aleppo began on Thursday and continued overnight, with thousands of people leaving in buses and ambulances.
But on Friday morning it was abruptly suspended, with the government accusing rebels of violating the terms of the accord.
"The evacuation operation has been suspended because the militants failed to respect the conditions of the agreement," a security source told AFP.
State television said rebels had tried to smuggle heavy weapons and hostage out of Aleppo.
The International Committee of the Red Cross, which has supervised the evacuation, confirmed it was on hold.The opposition accused the government of suspending the operation in a bid to secure the evacuation of residents from Fuaa and Kafraya, two villages under rebel siege in northwest Syria.
The government and its other main ally Iran have reportedly sought to add an evacuation of civilians from the villages to the terms of the Aleppo deal.
The Syrian Observatory for Human Rights, a Britain-based monitor, also said the suspension appeared to be related to the two villages.
It said pro-government fighters had blocked the road out of Aleppo that the evacuation convoy had been using and rebels were refusing the evacuation of residents of Fuaa and Kafraya.
The evacuation of the last pocket of rebel territory in Aleppo had been scheduled to begin on Wednesday, but was delayed by a day because of government objections, including over Fuaa and Kafraya.
Clashes resumed for several hours before the deal was revived, and on Thursday afternoon buses and ambulances began transporting evacuees to rebel territory in the west of Aleppo province.
The Observatory estimated some 8,500 people had left before the operation was suspended, including around 3,000 rebels.
Syrian state media reported a figure of around 8,000.Ahmad al-Dbis, a doctor helping coordinate the evacuation of the wounded, said at least 500 evacuees had wounds or illnesses requiring attention.
From a gathering point near the town of Khan al-Aasal, people were travelling further west, heading either to camps for the displaced or to stay with relatives or friends.
The departures began a month to the day after government forces launched a major offensive to retake all of Aleppo and secure the regime`s biggest victory in more than five years of conflict.
In a video message to Syrians on Thursday, President Bashar al-Assad said the "liberation" of Aleppo was "history in the making".
But US secretary of state John Kerry said what had already happened in the city was "unconscionable", raising concerns for the "tens of thousands of lives that are now concentrated into a very small area of Aleppo".
The UN Security Council will meet later Friday to discuss a French call for international observers to monitor the evacuation and ensure aid deliveries.
It is unclear how many people remain in east Aleppo, with tens of thousands fleeing to territory held by the government or Kurds in recent days, but perhaps tens of thousands are still inside.
The evacuations have been emotional for departing residents desperate for relief after months of bombardment and siege, but also tearful at the prospect of potentially permanent exile.
In the dust of one bus window Thursday someone had traced: "One day we will return".
"The scene was heart-breaking," said ICRC Syria head Marianne Gasser.
"People are faced with impossible choices. You see their eyes filled with sadness."
More than 310,000 people have been killed since the conflict began with anti-government protests in 2011, and over half the population has been displaced, with millions becoming refugees.
On Friday, Russian President Vladimir Putin said Moscow was "actively negotiating" with rebels via Turkish mediators.
"The next step (after Aleppo) will be to reach agreement on a complete ceasefire across all of Syria," he said in Japan.
He added that the parties were proposing fresh peace talks, possibly in Kazakhstan.