The United Nations' High Commissioner for Human Rights has accused the Security Council of emboldening the Syrian government to launch what she describes as an all-out assault to crush the opposition.
Navi Pillay told the UN general assembly on Monday the longer the international community delayed action, the longer Syria's civilian population would suffer.
Ms Pillay told the meeting in New York the scale of violence unleashed against opponents of Syrian president Bashar al-Assad suggests that crimes against humanity have been committed, the BBC reports.
She told the UN the lack of agreement encouraged Damascus to use "overwhelming force" against protests.
She also said widespread torture and rape had been carried out by security forces in Syria.
"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."
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.
Navil Pillay referred to the decision of Russia and China earlier in February to veto a UN Security Council resolution calling for Mr Assad to step down.
She 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".
Syria's envoy at the UN Dr Bashar Ja'afari described her comments as negative and unprincipled.
Human rights groups say more than 7000 have died throughout Syria since the uprising began in March last year. The government says at least 2000 members of the security forces have been killed combating "armed gangs and terrorists".
Meanwhile, Russia says it will consider a plan to send a joint UN-Arab peacekeeping force to Syria. However, it says there must be a ceasefire on the ground before troops can be sent in.