The U.N.'s human rights chief called Tuesday for an immediate humanitarian cease-fire in Syria, saying the situation in the country has deteriorated rapidly in recent weeks as authorities reinforce their onslaught against the opposition.

Navi Pillay, the U.N. High Commissioner for Human Rights, said the international community has to take action to prevent Syrian security forces from continuing their attacks against civilians, which she said had resulted in "countless atrocities."

"There must be an immediate humanitarian cease-fire to end the fighting and bombardments," Pillay told an urgent meeting of the U.N. Human Rights Council.

She urged Syria to end all fighting, allow international monitors to enter the country and give unhindered access for aid agencies to enter Homs and other embattled cities.

The appeal prompted a bitter riposte from Syria's ambassador to the United Nations in Geneva, who accused the 47-nation council of promoting terrorism in his country.

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Before walking out of the room, Fayssal al-Hamwi said Tuesday's meeting would only prolong the crisis in his country, where the United Nations estimates at least 5,400 people have been killed since March. Anti-government activists put the real figure much higher.

Pillay cited the report of a U.N. expert panel last week, which concluded that Syrian government officials were responsible for "crimes against humanity" committed by security forces against opposition members. The crimes included shelling civilians, executing deserters and torturing detainees. Some opposition groups, too, had committed gross abuses, it said.

The panel has compiled a confidential list of top-level Syrian officials who could face prosecution over the atrocities.

Pillay reiterated her call for Syria to be referred to the International Criminal Court "in the face of the unspeakable violations that take place every moment."

"More than at any other time, those committing atrocities in Syria have to understand that the international community will not stand by and watch this carnage and that their decisions and the actions they take today ultimately will not go unpunished," she said.

Members of the council are expected to pass a resolution Tuesday condemning "widespread and systematic violations of human rights and fundamental freedoms by the Syrian authorities."

A draft resolution supported by many Arab and Western nations says the regime's use of heavy artillery and tanks to attack civilian areas has contributed to the deaths of thousands of people since March.