Syrian forces are "systematically" torturing children held in detention, the UN human rights commissioner, Navi Pillay, has told the BBC.
Reports have also reached the UN that the rebel Free Syrian Army (FSA) is using child soldiers in its fight against President Assad's forces, another UN official said.
In the BBC interview, Pillay said Assad "could simply issue an order to stop the killings and the killings would stop".
An expert in war crimes and a former International Criminal Court judge, Pillay was damning in her denouncement of the Assad regime. She went so far as to claim there is enough evidence to refer Syria to the ICC.
"They've gone for the children - for whatever purposes - in large numbers. Hundreds detained and tortured... it's just horrendous," she told the BBC.
"Children shot in the knees, held together with adults in really inhumane conditions, denied medical treatment for their injuries, either held as hostages or as sources of information."
The Free Syrian Army opposition has also been hit by criticism of using children in the year-long conflict.
"We are receiving allegations of children with the Free Syrian Army," said Radhika Coomaraswamy, who is the UN representative for children and armed conflict.
Officials "haven't been able to verify or check" those reports, she admitted.
Despite the criticism the Syrian opposition appeared to be making efforts to codify its aims and objectives, after a meeting in the Turkish capital Istanbul where several groups agreed to work with the Syrian National Council.
But reports from inside the meeting claimed that walkouts and disagreements were rife.
The Free Syrian Army opposition fighting force has been described as a "brand" without a cohesive identity, according to a memo released last week by the US-based Saban Center.
Divisive tensions still exist among the roughly 1,200 rebel fighters who controlled the Baba Amr region of Homs before they were forced to leave following a month-long bombardment by the government.
The Global Post reports that several brigades among the FSA each accuse the other of "failing" the Syrian people.
Dozens of civilians in Homs were killed, some in apparent executions by government militias, after the FSA withdrew.
On Tuesday Assad supposedly agreed to a six-point plan for an end to the violence, as drawn up by UN and Arab League special envoy Kofi Annan.
The Iranian government also backed the plan on Wednesday, while also arguing for a "patient" approach to the crisis.
Russia and China have also endorsed the plan, which calls for a ceasefire, access to humanitarian aid and a political dialogue towards reform - while not mentioning Assad's personal grip on power.
Annan will supposedly travel to Damascus next week to discuss the implementation of the six-point proposal.
Even as the tentative moves towards peace appeared to gather momentum there were several reports of shelling and violence taking place across the country.
Al Jazeera English said that up to 600 Syrians fled from Deraa to Ramtha in Jordan on Tuesday night, even after government forces were reported to have crossed the border themselves in pursuit of the fleeing civilians.
The Syrian Strategic Research & Communication Centre said that 23 civilians were "executed" and 52 houses burned in the province of Idlib.
Avaaz quoted on resident of Homs as saying an elderly man was shot and killed why attempting to collect his pension.
An eyewitness said a sniper at the Department of Examinations hitting Ahmad Al-Fahd three times in the leg.
He said: "We rescued him by pulling him with the help of an iron rod because the sniper fired at us when we tried to reach him.
"We took him from house to house through alleyways but by the time we reached the field hospital he had lost a lot of blood and died. Cases like these happen every day in Homs, snipers haven’t left us a safe street to move on."
Elsewhere a leaked cache of emails sent between President Assad, members of his family and other figures from his inner circle, continued to embarrass the regime.
According to the Telegraph several messages showed Assad apparently mocking conservative Muslims.
One email from a female adviser to the president contains a joke in which a lost child can't describe his burka-wearing mother because he has "never seen her".
More than 9,000 people have been killed since the start of the uprising in March 2011 according to the UN.