The evacuation of east Aleppo has been halted, with reports of shooting and rebels accused of failing to respect a deal to lift sieges of their own.
At least 6,000 civilians and rebels are said to have left Aleppo, Syria's largest city, since Thursday after the government recaptured most of the city.
Among the evacuees are at least 2,700 children, a UN agency reports.
But thousands of cold and hungry civilians remain stranded in the rebel-held east, waiting to be picked up.
"Aleppo is now a synonym for hell," UN Secretary-General Ban Ki-moon told reporters in New York, urging a resumption of the evacuation.
US President Barack Obama accused Syrian President Bashar al-Assad and his allies Russia and Iran of "atrocities", saying the world was "united in horror" at the situation.
He admitted he felt responsibility for the situation but defended what he called his country's "best course" over the crisis.
France is calling on the UN Security Council to ensure the operation is co-ordinated by international observers, with humanitarian aid allowed into the city and hospitals given protection.
Four years of vicious fighting in eastern Aleppo left thousands dead, destroyed hospitals and wiped out food supplies.
The battle to control Aleppo may have reached its end, but there is still a fight over the process meant to rescue civilians while it also brings out the combatants, the BBC's chief international correspondent, Lyse Doucet, reports from Beirut.
Why did the evacuation stop?
Confusion reigned on Friday morning when the evacuation, which was taking place along corridors out of Aleppo towards rebel-held areas (Khan al-Asal and Khan Touman), was stopped.
Vehicles came under fire from pro-government militias, according to activists in the area, and their convoy returned to a rebel enclave.
The opposition Free Syrian Army says it still has 6,000 fighters in the city and they will fight to the end if they have to.
Russia insisted the evacuation was complete, saying 9,500 people had been brought out.
"All peaceful civilians and a majority of militants have left the blockaded districts," Lt-Gen Sergei Rudskoi told reporters.
However, Turkey, which supports the FSA, contradicted this, saying many people still wanted to leave.
Speaking in Ankara, Turkish Foreign Minister Mevlut Cavusoglu said consultations were under way with Russia and Iran, as well as "elements on the ground".
An unnamed Syrian official overseeing the operation told AFP news agency the evacuation had been suspended "because the militants failed to respect the conditions of the agreement".
Under a deal brokered by Russia and Turkey, the rebels were meant to ease their siege of towns in Idlib province.
Reports suggest the biggest rebel group in Idlib, the Jabhat Fateh al-Sham jihadists formerly known as the Nusra Front, have now agreed to allow injured people to leave the towns, Foua and Kefraya, which are home to some 20,000 people.
Syrian state media also accused rebels in Aleppo of trying to smuggle captives and heavy weapons out with them as they left Aleppo.
Who is still trapped in eastern Aleppo?
The UN's children's charity Unicef says sick and wounded children are among the evacuees from east Aleppo, some of whom left without their parents.
"However, hundreds of other vulnerable children, including orphans, remain trapped inside that part of the city," it added.
"We are extremely concerned about their fate. If these children are not evacuated urgently, they could die."
Abdulkafi al-Hamdo, a teacher who is also still in east Aleppo with his young daughter, told the BBC by phone he did not want to leave his home and city but believed he had no choice.
"The weather is so cold," he said. "Some people have been here since nine AM yesterday and the children are so hungry they are crying. They are freezing. Most of them here are scared of a brutal end to the ceasefire.
"They are afraid that they will not be able to get out. This is the feeling of most people here."
On Thursday, the UN envoy to Syria, Staffan de Mistura, had put the number of people remaining in east Aleppo at 50,000.
Is there any sign of peace in Syria?
Government forces have captured almost all of the rebel-held territory in eastern Aleppo, with the help of Russian air strikes. But the conflict in Syria is far from over.
Friday saw new protests across the world, from Malaysia to Turkey, to condemn violence against Syrian civilians and show solidarity with the country's opposition.
Russian President Vladimir Putin called for all sides to agree to a complete nationwide ceasefire.
In another development on Friday, Syrian state media report that a girl aged between seven and nine, wearing a bomb belt, blew herself up at a police station in the Midan district of the capital, Damascus.
Three police officers were reportedly injured in the explosion.