The Syrian regime had been "emboldened" by the UN security council's failure to condemn violence being committed against its civilians, the organisation's human rights chief said.
Navi Pillay, the UN's High Commissioner for Human Rights, said at a meeting of the General Assembly that the Syrian regime was guilty of "systematic human rights violations".
President Bashar al-Assad's forces have used schools as "detention facilities, sniper posts and military bases," she said, and have killed more than 400 children.
Children have been arrested and subjected to solitary confinement, she said, and many cities have been blockaded.
Supplies of water, food and medicine are all being blocked from reaching civilians, she said.
Pillay added that it was "likely" crimes against humanity had been committed, and implored the international community to take action.
A resolution condemning the violence was vetoed in January by China and Russia.
At the General Assembly meeting held to discuss the situation in Syria, members discussed a 2011 report by the UN Human Rights Council which detailed abuses by Syrian authorities against protesters.
The report, released in November, said that the regime had killed more than 250 children.
Syrian force reportedly resumed shelling the beleaguered city of Homs on Monday for a 10th straight day.
Activists have said at least 23 people were killed on Sunday alone, as protests sprung up in several other areas of Syria to show support for the embattled citizens of Homs.
Also on Sunday the Arab League asked the UN to lead a peacekeeping mission to the country, but Russia said that a ceasefire was needed before any peacekeeping mission could theoretically begin.
"We should first have peace," said the Russian foreign minister, Sergei Lavrov, according to the New York Times, blaming "out of control" opposition groups for the violence.
The Syrian ambassador to the Arab League said he was "not interested" in any decision made by the group, according to state Syrian news network Sana.
At least 5,000 people have been killed since the uprising began in March 2011, the UN has said.