Syria's army on Tuesday urged the last remaining rebels and civilians to leave the besieged opposition enclave in Aleppo as it prepares to take full control of the devastated city.
The evacuation of Aleppo's rebel sector is seen as a pivotal moment in the nearly six-year war that has killed more than 310,000 people and triggered a major humanitarian and refugee crisis.
The government and other parties on the ground have agreed to allow 20 observers to be sent to east Aleppo to monitor evacuations, said UN spokesman Stephane Dujarric.
Following a flurry of diplomatic activity, key regime allies Russia and Iran as well as rebel supporter Turkey agreed to act as guarantors in Syria peace talks, Russian Foreign Minister Sergei Lavrov said earlier.
They also backed expanding a ceasefire, according to Lavrov, who said the evacuation of east Aleppo should finish within "one or two days".
In east Aleppo, soldiers using megaphones called on the remaining fighters and civilians to exit the opposition districts, a military source told AFP.
"The army is expected to enter (Aleppo) to clean the area after the fighters leave," the source said.
At least 25,000 people have left rebel districts of Aleppo since the operation began last week, according to the International Committee of the Red Cross, which is overseeing the operation.
Spokeswoman Ingy Sedky said 750 people had been evacuated in parallel from Fuaa and Kafraya, two Shiite-majority villages in northwest Syria besieged by rebels, as part of the deal.
Ten buses left the battered east on Tuesday morning, Sedky told AFP.
But no other buses moved the rest of the day, according to the Syrian Observatory for Human Rights and a medical official overseeing evacuations.
- Waiting for evacuees -
Observatory head Rami Abdel Rahman said the delay was due to complications in Fuaa and Kafraya.
Around 20 buses had entered the two villages on Tuesday but had yet to evacuate anyone, Abdel Rahman said, but he had no details on the reason.
State news agency SANA blamed the "delay in the last batch" of evacuations from Aleppo on "disputes" among rebel groups.
"I'm waiting for my brother to get out of east Aleppo -- he's a 70-year-old civilian," Nuhad al-Seikh told AFP just outside rebel-held Aleppo.
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"I've been speaking to him over the cellphone. He said he boarded a bus but is still waiting," said the 55-year-old.
An AFP correspondent at Khan al-Assal, the staging ground where evacuees from Aleppo, Fuaa and Kafraya were being transferred to their respective destinations, saw hundreds of people aboard eight buses from the two villages.
Bashar Babbour, who works with the medical charity UOSSM, told AFP from Khan al-Assal that evacuees from Tuesday morning looked cold and exhausted.
"When people are going to the medical assessment teams, most of them ask for food and water, in addition to blankets, especially if they're arriving at night," Babbour said.
He said many other evacuees were also asking if there was internet access there so they could contact friends or relatives.
- Sick and wounded -
The World Health Organization said the Aleppo evacuees included around 300 needing medical treatment, among them dozens of children.
"The huge majority of these patients have trauma injuries," spokesman Tarik Jasarevic said, adding that 93 people in critical condition had been referred to hospitals in Turkey.
Aleppo was once Syria's commercial and industrial hub, but it has been divided since 2012 between government forces in the west and rebels in the east.
Government forces launched an offensive in mid-November to capture the whole city, and had seized more than 90 percent of the eastern half when the evacuation deal was struck.
In a regime-held neighbourhood on the southern edges of Aleppo, an AFP correspondent saw Syrian troops strolling through the area on Tuesday while others gathered around burning tyres to stay warm.
"I hope today is the last day of rebels leaving, because we've been promised we can go on vacation and rest if the operation ends today," said Akram, a soldier.
Diplomatic efforts -- including several rounds of peace talks in Geneva -- have failed to resolve the conflict, which reached a turning point last year when Russia launched air strikes in support of Assad.
In a rare show of international unity, the UN Security Council on Monday unanimously adopted a French-drafted resolution to monitor the Aleppo evacuations.
UN envoy Staffan de Mistura also said he hoped to convene new peace talks in Geneva in February.
In a boost to the troubled peace process, the foreign ministers of Russia, Iran and Turkey agreed Tuesday on "the importance of widening the ceasefire, of free access for humanitarian aid and movement of civilians onto Syrian territories".
Turkish President Recep Tayyip Erdogan meanwhile said he had agreed with Russian counterpart Vladimir Putin in a phone call that the assassination of Moscow's ambassador in Ankara on Monday would not harm cooperation including on Syria.