More than 4,000 people have been killed during a military crackdown on protesters that started in March and more than 14,000 people are believed to be in detention, said Navi Pillay, the UN High Commissioner for Human Rights.
"In light of the manifest failure of the Syrian authorities to protect their citizens, the international community needs to take urgent and effective measures to protect the Syrian people," Ms Pillay told an emergency session of the U.N. Human Rights Council.
Ms Pillay did not spell out what measures world powers should take – Western leaders have in the past shied away from suggestions of military action, along the lines of the Nato campaign that helped unseat Libyan leader Muammar Gaddafi.
The United States, the EU, members of the Arab League and neighbouring Turkey have already imposed sanctions on the Damascus government.
After she spoke, envoys from Russia and China, which both have oil projects in Syria, took the floor to warn against foreign interference in Syria in the name of human rights.
Both countries blocked international efforts to condemn the government of Syrian President Bashar al-Assad by vetoing a U.N. Security Council resolution in October.
"We would like to warn against illegal interference by outside forces even under the pretext of protecting human rights," said Russia's envoy Valery Loshchinin. "This will have serious and unforeseen consequences."
Mr Loshchinin called on all sides in Syria to halt violence.
"We hear that the conflict in Syria continues to be fuelled by outside forces, armed and terrorist groups being organised and supplied with weapons and money from abroad."
Russia has delivered anti-ship cruise missiles to Syria, the Interfax news agency cited an unnamed military source as saying on Thursday, days after a UN commission of inquiry called for an arms embargo on Damascus.
China's envoy He Yafei said that although China was deeply concerned, views on how to resolve the situation differed widely.
"Member states of the United Nations should follow the principles and purposes of the UN and refrain from resolving differences through force or threat of force," he said.
Ms Pillay said 307 children had died in the conflict, up from an estimate of 256 that was released earlier this week.
"The Syrian authorities' continual ruthless repression, if not stopped now, can drive the country into a full-fledged civil war ... All acts of murder, torture and other forms of violence must be immediately stopped," she added.
She said she was also concerned about reports of increased armed attacks by the opposition forces, including the so-called Free Syrian Army, against the Syrian military and security apparatus.
A UN commission of inquiry said this week it had found solid evidence of crimes against humanity by security forces, including executions, torture and rapes.
US human rights Ambassador Eileen Chamberlain Donahoe told reporters there was "evidence of the complicity in these atrocities" by Syrian authorities and said there was no doubt the violence was intentional.
"It's systematic, widespread and to the point of possibly rising to the level of crimes against humanity," she said.
Syria, which does not allow access to most foreign journalists, says it is fighting an insurgency by armed groups supported from abroad, who have attacked its troops trying to defend the peace.
At Friday's summit, Syria's ambassador Faysal Khabbaz Hamoui gave an angry speech that won support from Russia, China and Cuba.
"The Syrian problem is one that can be resolved only by Syrians. It is only a domestic, national solution that is possible," he said, referring to reforms promised for 2012.
"The solution cannot come from the corridors of the international community," he said. "It is only resolutions trying to put more oil on the fire."
A revised EU draft resolution – which emerged after intense negotiations with China, Cuba and Russia – condemns "continued, widespread, systematic and gross violations of human rights and fundamental freedoms".
The aim is to get all countries to agree on a strong message of disapproval to Damascus coupled with a call for a halt to violence, diplomats said.