Evacuations Sunday of fighters and civilians from the devastated Syrian city of Aleppo were postponed until further notice after gunmen attacked buses for a similar operation from two rebel-besieged villages.
The development came as Syria ally Russia warned it would veto a French-drafted resolution at the Security Council on sending United Nations observers to Aleppo and submitted a counter draft resolution.
Diplomats said the Security Council vote would take place on Monday.
Dozens of buses had entered the last rebel-held parts of Aleppo on Sunday to resume the evacuation of thousands of increasingly desperate trapped Syrian civilians and rebels.
The evacuation was suspended on Friday, a day after convoys of people had begun leaving the rebel sector under a deal allowing the regime to take full control of the battleground city.
The main obstacle to a resumption had been a dispute over how many people would be evacuated in parallel from two Shiite villages, Fuaa and Kafraya, under rebel siege in northwestern Syria.
But just as a deal to go ahead with the evacuations was found and announced by both sides, gunmen attacked buses sent to take people out of Fuaa and Kafraya and torched them, a monitor said.
The Syrian Observatory for Human Rights said one bus driver was killed in the attack and that the overall evacuation operation was put on hold.
SANA/Handout via Reuters
The Britain-based monitor said security guarantees were needed before they could resume.
Yasser al-Youssef of the Nureddin al-Zinki rebel group confirmed that "the evacuations have been momentarily suspended."
The Observatory said buses would not leave the rebel areas of Aleppo until residents of Fuaa and Kafraya were also able to leave.
Buses began entering several east Aleppo districts earlier Sunday under Red Crescent and International Committee of the Red Cross (ICRC) supervision "to bring the remaining terrorists and their families out," state news agency SANA said, referring to the rebels.
State television said 100 buses would take people out.
By early evening in Aleppo, more than 30 buses were packed with people awaiting evacuation, while thousands more stood in the cold for their turn to board other buses, an AFP reporter said.
A rebel representative had also said that hundreds of people would also be evacuated from Zabadani and Madaya, two regime-besieged rebel towns in Damascus province, as part of the deal.
The UN Security Council met to discuss a French draft resolution saying that "tens of thousands of besieged Aleppo inhabitants" are in need of aid and evacuation.
The measure would task Secretary General Ban Ki-moon with deploying UN staff to Aleppo to monitor evacuations and report on the protection of civilians who remain there.
But Russia said it would use its veto to block the French proposal, and instead presented a rival draft asking the UN to make "arrangements" to monitor the situation.
The Russian draft seen by AFP made no specific mention of observers going to Aleppo.
"We believe quite simply that what they are proposing is unworkable and dangerous," Russian Ambassador Vitaly Churkin said ahead of the meeting.
"We cannot allow it to pass because this is a disaster."
The Security Council will vote Monday on a new draft resolution on observers after France agreed to take Russian concerns into account, diplomats said.
Russian President Vladimir Putin's special envoy to Syria visited Damascus ally Iran on Sunday for talks with top officials on the Syrian conflict.
Iran's official news agency IRNA said the foreign ministers of Russia, Turkey and Iran would meet Tuesday in Moscow to discuss the situation.
Families have been sheltering at night in freezing temperatures in bombed out apartment blocks in Aleppo's Al-Amiriyah district, the departure point for evacuations.
An AFP reporter visited a hospital in the rebel sector where patients lay on floors without food or water and almost no heating.
Aleppo has seen some of the worst violence of the nearly six-year war that has killed more than 310,000 people.
A physiotherapist, Mahmud Zaazaa, said only "three doctors, a pharmacist and three nurses" remain in the area.
An official said more than half of Aleppo's buildings and infrastructure have been badly damaged or destroyed since violence erupted there in 2012.
"This is an optimistic percentage of the damage," Aleppo administrator Nadeem Rahmoun said.
UN envoy Staffan de Mistura estimated that as of Thursday around 40,000 civilians and perhaps as many as 5,000 opposition fighters remained in Aleppo's rebel enclave.
The ICRC appealed for safe passage for the civilians.
"People have suffered a lot. Please come to an agreement and help save thousands of lives," said Syria delegation head Marianne Gasser.
Before evacuations were suspended around 8,500 people, including some 3,000 fighters, left for rebel-held territory elsewhere in the north, said the Observatory.
By AFP's Karam al-Masri in Aleppo, Syria, with Layal Abou Rahal in Beirut, Lebanon.