Saturday's blast hit a convoy carrying residents from the northern towns of Fuaa and Kafraya as they waited at a transit point in rebel-held Rashidin, west of Aleppo.
At least 68 children were among the 126 people killed in the attack, the Syrian Observatory for Human Rights said, updating a previous toll of 112 dead.
At least 109 of the dead were evacuees, the Britain-based monitoring group said, while the rest were aid workers and rebels guarding the convoy.
The evacuations were taking place under a deal between Syria's regime and rebels that is also seeing residents and rebels transported out of Madaya and Zabadani, towns near Damascus which are surrounded by pro-government forces.
The agreement is the latest in a string of evacuation deals, which the government of President Bashar al-Assad says are the best way to end the violence after more than six years of civil war.
Rebels say they amount to forced relocations after years of bombardment and crippling sieges.
Body parts and the belongings of evacuees -- including clothes, dishes and even televisions -- were still strewn at the scene of the attack on Sunday, an AFP correspondent said.
The shattered buses were nearby as was the shell of a pick-up truck -- with little left but its engine block -- that was apparently used to carry out the bombing.
There was no immediate claim of responsibility for the bombing, though the key Ahrar al-Sham rebel group denied any involvement. The government blamed "terrorists" -- a catch-all term for its opponents.
The Observatory, which relies on a network of sources inside Syria to monitor the conflict, said hundreds of people were also wounded in the blast.
'PEOPLE CRYİNG AND SHOUTİNG'
It said a petrol station at the transit point was caught up in the explosion, adding to the number of victims.
The Syrian Red Crescent said three of its workers were among the wounded.
Maysa al-Aswad, a 30-year-old evacuee from Kafraya, said she was sitting on one of the buses with her six-month-old son Hadi and 10-year-old daughter Narjis when the blast shook the parked convoy.
"Hadi was on my lap and Narjis on a chair next to me. When the explosion happened I hugged them both and we fell to the floor," she told AFP by telephone from near Aleppo.
"I didn't know what was happening, all I could hear was people crying and shouting," she said.
"All I can think about is how we survived all the death during the last few years and then could have died just after we finally escaped."
More than 5,000 people left Fuaa and Kafraya and about 2,200 left Madaya and Zabadani on Friday, the latest in a series of evacuations from the four towns under the agreement.
The evacuation process resumed after the bombing, the Observatory said, with the residents of Fuaa and Kafraya eventually arriving in Aleppo, Syria's second city which the government gained full control of last year.
Wounded survivors, including many children, were taken for treatment at an Aleppo hospital.
'MONSTROUS, COWARDLY ATTACK'
UN aid chief Stephen O'Brien condemned the bombing, saying in a statement: "The perpetrators of such a monstrous and cowardly attack displayed a shameless disregard for human life."
Pope Francis on Sunday also urged an end to the war in Syria as he presided over the traditional Easter mass in Rome.
The pontiff said he hoped that Jesus Christ's sacrifice might help bring "comfort and relief to the civil population in Syria, prey to a war that continues to sow horror and death".
The residents and rebels from Madaya and Zabadani arrived late Saturday in rebel-held territory in Idlib province, where they were greeted with embraces and shots fired into the air.
It was not immediately clear whether further evacuations were taking place on Sunday.
The evacuation deal was brokered by Qatar, a longtime supporter of Syrian opposition forces, and Iran, a key regime ally.
Shiite-dominated Iran has repeatedly raised concerns for the residents of Fuaa and Kafraya, who are mainly Shiites and were besieged by Sunni rebels.
Syria's war has left more than 320,000 people dead since erupting in 2011, with more than half the population forced from their homes and hundreds of thousands trapped under siege.
