People who were evacuated from the two villages of Kefraya and al-Foua walk near buses, after a stall in an agreement between rebels and Syria's army, at insurgent-held al-Rashideen, Aleppo province, Syria April 15, 2017.
Also Saturday, an Iraqi military statement said the Syrian air force has carried out a series of airstrikes against Islamic State group militants inside Syria, with one killing a leading member of the group in Raqqa, the IS de facto capital.
At least 39 people are dead after a vehicle bomb exploded near a convoy carrying evacuees from besieged areas of Syria, human rights groups and Syrian media report.
Meanwhile residents from the Shiite villages were still waiting in insurgent territory on Aleppo's outskirts to cross into the city, the Observatory and a witness said.
The convoy of almost 5,000 people had been stuck in Rashidin on the outskirts of Aleppo after a population transfer deal with rebel forces stalled.
The British-based Syrian Observatory for Human Rights reported casualties, saying the explosion appeared to be caused by a bomb. Thousands of evacuees from the pro-government and opposition areas were stuck opposite sides of the edge of Aleppo city as rebels and government bickered over the terms for evacuating fighters.
It shattered coaches and set cars on fire, leaving a trail of bodies, as the convoy waited at a marshalling point in rebel-held territory at Rashidin.
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Syrian state TV blamed the rebels for obstructing the deal, causing thousands of evacuees to be stuck in bus depots overnight.
A Lebanese TV channel close to the Syrian government is airing footage showing new buses arriving at an evacuation point that was site of a major explosion that killed dozens.
Earlier in the day, a source told Sputnik that a suicide attacker detonated a auto bomb in the Rashidin neighborhood of Aleppo near a convoy of buses carrying civilians evacuated from the Syrian Shiite towns of Fua and Kefraya.
The Syrian Arab Red Crescent distributed food and water to the waiting passengers, who included 3,700 civilians, at Rashidin, west of government-held second city Mosul.
The evacuees from Madaya were expected to head to rebel-held Idlib, 30 miles (48km) south-west of Aleppo.
The exact reasons for the delay in completing the evacuation deal were also unclear. "The bus garage is small so there's not much space to move around", Ahmed, 24, said.
A resident of Zabadani - another rebel-held town to be evacuated - Amer Burhan says no evacuation had even taken place from there.