Navi Pillay told the UN the lack of agreement encouraged Damascus to use “overwhelming force” against protests.
Activists say more than 400 people have been killed since security forces launched an assault on opposition-held areas in the city of Homs this month.
Earlier, the Arab League called for a joint Arab-UN peacekeeping force.
Ms Pillay, the UN High Commissioner for Human Rights, referred to the decision of Russia and China earlier this month to veto a UN Security Council resolution calling for Syrian president Bashar al-Assad to step down.
She told UN delegates in New York: “The failure of the Security Council to agree on firm collective action appears to have emboldened the Syrian government to plan an all out assault in an effort to crush resistance with overwhelming force.
“I am particularly appalled by the ongoing violence in Homs.”
She added that the humanitarian situation in Homs was “simply deplorable”.
Ms Pillay said the UN Human Rights Council had attempted to keep track of the number of fatalities reported in the Syrian uprising, although in the past two months this task had become “almost impossible”.
However she said: “We are certain that the number of dead and injured continues to rise every day.”
Bashar Ja’afari, Syria’s permanent representative to the UN, insisted that al-Assad’s government was fighting armed gangs who want to destabilise the country.
He said: “Buildings have been mined by some of these terrorist groups.
“This isn’t peaceful demonstration – this is violence.”
The EU had earlier on Monday backed the Arab League’s “bold” plan for a peacekeeping force to be sent to Syria but Russia said violence must end first.
Foreign Minister Sergei Lavrov said there had to be a ceasefire in place before any peacekeepers could be sent.
But he added: “The problem is that the armed groups that are fighting the Syrian regime do not answer to anyone and are not controlled by anyone.”
China, meanwhile, said Syria’s problems needed to be resolved by diplomatic means.
Foreign ministry spokesman Liu Weimin said: “China hopes all relevant parties can keep dialogue and communication to play a positive and constructive role in politically resolving the Syrian issue and easing the country’s tension.”
Speaking before the assembly meeting started while on a visit to South Africa, UK Foreign Secretary William Hague said: “I don’t see the way forward in Syria as being Western boots on the ground, in any form, including in peacekeeping form, but of course if such a concept could be made viable we will be supporting it in all the usual ways.”
Human rights groups say more than 7,000 have died throughout Syria since last March.
The government says at least 2,000 members of the security forces have been killed combating “armed gangs and terrorists”.
However, Syria restricts access to foreign media and it is not possible to verify casualty figures.