The failure of the UN security council to take action has emboldened Syria to make an "all out assault" on opponents, the UN's human rights chief says.
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."
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.
Bashar Ja'afari, Syria's permanent representative to the UN, described Ms Pillay's comments as negative and unprincipled and insisted that Mr 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."
But US Secretary of State Hillary Clinton also accused Syria of stepping up attacks on civilians.
"It is deplorable that the regime has escalated violence in cities across the country, including using artillery and tank fire against innocent civilians. We stand with the Syrian people and we are looking for a peaceful resolution," she told a news conference.
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.
Michael Mann, spokesman for EU foreign policy chief Catherine Ashton, said: "We welcome these bold decisions and the strong and clear commitment and leadership that the Arab League is taking to resolve the crisis in Syria.
"The EU's first goal is an immediate cessation of killings and therefore we are very supportive of any initiative that can help achieve this objective, including a stronger Arab presence on the ground in co-operation with the UN to achieve a ceasefire and the end of violence."
He added: "We renew our urgent calls on all members of the Security Council to be constructive and act with responsibility at this crucial moment."
Russian 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."