The United Nations' human rights chief urged world powers on Friday to take action to protect civilians in Syria from ruthless repression, but her call was swiftly criticised by envoys from China and Russia.

More than 4,000 people have been killed during a military crackdown on street protesters that started in March and more than 14,000 people are believed to be in detention, said U.N. High Commissioner for Human Rights Navi Pillay.

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, Pillay told an emergency session of the U.N. Human Rights Council in Geneva.

She 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 rebels unseat Libyan leader Muammar Gaddafi in August.

The European Union called for pro-active action by the international community, while Kuwait's envoy said that there may be a need for intervention to safeguard civilians.

The United States, the EU, members of the Arab League and neighbouring Turkey have already imposed sanctions on Syria over Assad's failure to implement commitments to withdraw tanks and troops from restive cities and start talks with his opponents.

After Pillay 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.

The Russian-Chinese stance, in turn, drew fierce criticism.

In the face of brutal repression occurring in Syria, it is outrageous that some governments continue to obstruct efforts here and elsewhere in the United Nations to bring an end to these crimes against humanity, Peter Splinter of Amnesty International told the talks, singling out permanent members of the Security Council, an allusion to Russia and China.

It is now time for the U.N., including the Security Council, to deliver an effective international response to Syria's human rights crisis, he said.

CHINA, RUSSIA WARNING

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.

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, a longstanding arms supplier to Assad, has now delivered anti-ship cruise missiles to Syria, the Interfax news agency cited an unnamed military source as saying on Thursday.

This was shortly after a U.N. commission of inquiry called for an arms embargo against 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 U.N. and refrain from resolving differences through force or threat of force, he said.

CRIMES AGAINST HUMANITY

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.

A U.N. commission of inquiry said this week it had found solid evidence of crimes against humanity by security forces, including executions, torture and rapes.

U.S. 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.

Syria, which bars 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 the U.N. Human Rights Council meeting, Syrian 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...Only a domestic, national solution ... is possible, Syrian Ambassador Faysal Khabbaz Hamoui told the rights council meeting, referring to reforms Assad has promised for 2012.

The solution cannot come from the corridors of the international community, he said. (This) is only resolutions trying to put more oil on the fire.

(Editing by Mark Heinrich)