The agency didn't give further details about the footage, such as whether it shows the explosion in Rashidin, a suburb of Aleppo in northwestern Syria.
But the statement fits with other reports, including one from a witness who said many of those killed or injured in the attack were children who were picking up handout snacks.
No one has claimed responsibility for the blast, in which 126 people -- including 68 children -- were killed, according to the Syrian Observatory for Human Rights monitoring group. Dozens of others were injured, according to Syria Civil Defense, the volunteer rescue groups also known as the White Helmets.
"While at this stage (we're) unable to confirm how the attack was carried out or those responsible, footage seen by the UN human rights office showed children gathering around a person giving out sweets just prior to the explosion," Rupert Colville, spokesman for the UN High Commissioner for Human Rights, said in a written statement Tuesday.
CNN has not verified and is not showing the footage, which spread on social media.
Victims were part of evacuee swap
The explosion happened outside a line of parked buses that were evacuating hundreds of people from two regime-held but rebel-besieged villages in northwest Syria.
At least 109 of those killed were evacuees from the pro-regime Shia villages of Al-Fu'ah and Kafraya; the rest were aid workers and rebels guarding the convoy, according to the Syrian Observatory.
Rebels allowed the evacuees to leave their villages last week as part of a deal with the Syrian government, which the insurgents have been fighting for six years
As part of the deal, the government also allowed thousands of people to leave two rebel-held towns that regime forces had long besieged. Those towns were Madaya and Zabadani in southwestern Syria.
Both sets of evacuees headed to areas friendly to them. The deal, dubbed the Four Towns Agreement, was brokered by Iran and Qatar, Agence France-Presse reported.
Saturday's explosion happened as the Al-Fu'ah and Kafraya convoy, bound for a regime-controlled portion of Aleppo, paused at a checkpoint in Rashidin. Part of the convoy continued into Aleppo late Saturday, after the attack, SANA reported.
State-run media: Rigged vehicle was disguised
An explosives-rigged vehicle exploded near the parked convoy in Rashidin, the state-run Syrian Arab News Agency reported. The vehicle had been packed with children's food supplies, perhaps to disguise it, a SANA correspondent said.
It wasn't immediately clear whether the vehicle that exploded had anything directly to do with the snacks distribution that was reportedly happening at that moment.
Photographer and activist Abd Alkader Habak
said he was there filming someone who was "distributing snacks to some of the evacuees."
He said he stepped aside to talk with one of his colleagues, and then felt the explosion, which sent him to the ground.
"The majority of those killed or injured were children" who had gathered for the snacks, he said.
The attack likely amounts to a war crime, said Colville, the UNHCR spokesman. Intentional attacks on civilians are among war crimes listed in the Rome Statute of the International Criminal Cour
UN says some injured are effectively missing
Some of those who were injured in Saturday's attack are effectively missing, because they are "believed to have been taken by armed opposition groups to opposition-controlled hospitals" in Syria's Idlib governorate, Colville said.
"Due to their perceived sympathies for the government of Syria, their families are concerned for their safety," Colville said. He did not say how many injured people this applied to.