The top United Nations human rights official has told the Security Council that Syria should be referred to the International Criminal Court over its crackdown on anti-government protests.
Navi Pillay said she felt widespread killings and torture in the country "constituted crimes against humanity".
Ms Pillay put the number of those killed by security forces in the nine-month uprising at more than 5,000.
Syria's UN envoy said Ms Pillay was "not objective" and "not fair".
Ms Pillay, the UN High Commissioner for Human Rights, told a closed session of the Security Council that 300 children had been among those killed since the start of the uprising in March.
She also said 14,000 people were believed to have been arrested, and 12,400 had fled to neighbouring countries.
"It is based on the evidence and the widespread and systematic nature of the killings, the detentions and the acts of torture that I felt that these acts constituted crimes against humanity and I recommended that there should be a referral to the International Criminal Court," Ms Pillay said.
She said her estimate of more than 5,000 deaths did not include security forces. The Syrian government has said more than 1,000 of its police and troops have been killed.
The Syrian ambassador at the UN, Bashar Jaafari, said Ms Pillay had "allowed herself to be misused in misleading the public opinion by providing information based on allegations collected from 233 defectors".
He added: "How could defectors give positive testimonies on the Syrian government? Of course they will give negative testimonies against the Syrian government."
Call for action
It is difficult to confirm the exact casualty toll in Syria because there are no independent monitors on the ground and international journalists have been denied access to the country.
Ms Pillay said the protesters in Syria had remained largely peaceful since the uprising erupted in March, but that attacks against the Syrian government had been increasing.
Many Syrian army deserters have joined opposition forces in recent months.
Ms Pillay warned that inaction by the international community would only embolden the Syrian authorities.
The EU has imposed 10 rounds of sanctions on the Syrian government, and the Arab League has suspended its membership. However the UN has not so far passed a resolution condemning Damascus.
Russia and China both vetoed a European-led draft at the UN in October. India, South Africa and Brazil have also been reluctant to support action at the Security Council.
Ms Pillay urged the council to "speak coherently with one voice".
"Urgent, effective measures in a collective and decisive manner must be taken to protect Syrians," she said.
After meeting Ms Pillay, German Foreign Minister Guido Westerwelle said he was "really shocked about what I heard about the atrocities in Syria".
He said countries on the Security Council that were still hesitating to condemn Damascus had to change their mind.
But Russia's UN ambassador Vitaly Churkin complained that the body had recently seen key members "switch gears and turn into regime change mode".
"That of course is something which can not be conducive to a political process and that's what troubles us a great deal."
Fighting was said to have continued in several cities on Monday, with at least 20 people reported killed.
The Local Co-ordination Committees (LCC), a network of opposition activists, said the deaths had occurred in Idlib in the north, Homs and Hama further south, and in a suburb of Damascus.
Fierce fighting is also reportedly continuing in the southern province of Deraa.
Local elections were held across the country - part of President Assad's very slow and not entirely convincing reform programme announced some months ago, says the BBC's Jonathan Head in Istanbul, in neighbouring Turkey.
The opposition had called for a boycott.
In anti-government strongholds activists said there were few signs that an election was even happening, and almost no-one was voting, says the BBC's Jonathan Head in neighbouring Turkey.
The opposition called a general strike over the weekend, and much of the country has more or less shut down.