Barack Obama denounced the "horror" in Aleppo and called for impartial observers.
US President Barack Obama denounced the "horror" in Aleppo Friday and called for impartial observers after Syria's government suspended the evacuation of civilians and fighters from the last rebel-held areas.
Obama said President Bashar al-Assad's Syrian regime and its Iranian and Russian backers are solely responsible for the slaughter of civilians in Aleppo.
"The world as we speak is united in horror at the savage assault by the Syrian regime and its Russian and Iranian regime on the city of Aleppo," he told an end-of-year news conference.
"This blood and these atrocities are on their hands."
He warned Assad that he will not be able to "slaughter his way to legitimacy".
UN Secretary General Ban Ki-moon earlier warned that Aleppo had become "a synonym for hell" and said the United Nations urged "all necessary measures" for a safe resumption of the evacuation.
The UN Security Council could vote as early as this weekend on a French-drafted proposal to allow international observers in Aleppo and ensure urgent aid deliveries.
US Ambassador Samantha Power said after a closed-door council meeting that UN officials were ready to be sent quickly to Aleppo.
"The presence of independent observers can deter some of the worst horrors," she said.
But Russian Ambassador Vitaly Churkin argued that it would take weeks to send monitors to Aleppo.
Turkey's Foreign Minister Mevlut Cavusoglu spoke "13 times" on Friday with his Iranian counterpart Mohammad Javad Zarif to discuss the situation, a Turkish diplomatic source said.
Turkey and a Syrian military source said the evacuation had been suspended but was not yet over, while Moscow insisted the operation was "complete", with all women and children moved from the city.
Besieged VillagesAmid 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 International Committee of the Red Cross, supervising the evacuations, said it was looking into reports of shooting before the convoy was turned back.
The delicate operation to bring the last civilians and rebels out of east Aleppo began on Thursday and continued overnight, with thousands leaving in buses and ambulances.
But on Friday morning it was abruptly suspended, with the government accusing rebels of violating the terms of the deal.
"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 out heavy weapons and hostages.
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 Iran have reportedly sought to add an evacuation of civilians from the villages to the terms of the Aleppo deal.
The Britain-based Syrian Observatory for Human Rights said pro-government fighters had blocked the road the evacuation convoy had been using and rebels were refusing the evacuation of residents of Fuaa and Kafraya.
Evacuating the last rebel pocket in Aleppo had been scheduled to start 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 buses and ambulances began transporting evacuees to rebel territory in western Aleppo province.
Video messageThe 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 treatment.
From a staging area 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 Thursday, Assad said the "liberation" of Aleppo was "history in the making".
But US Secretary of State John Kerry said what had already happened there was "unconscionable", raising concerns for the "tens of thousands of lives that are now concentrated into a very small area of Aleppo".
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 traced: "One day we will return".
'Heart-breaking'
"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.
Also Friday, a blast at a Damascus police station was caused by a young girl carrying explosives, media reported.
State news agency SANA said the eight-year-old was sent in carrying "a bomb" which was detonated from a distance.
A police source told the Al-Watan daily that she asked to use the bathroom and the explosives detonated.
Diplomatic efforts -- including several rounds of peace talks in Geneva -- failed to make progress in resolving the conflict, which reached a turning point last year when Russia launched an air war in support of Assad.
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.