U.N. High Commissioner for Human Rights Navi Pillay, in statements before the U.N. Human Rights Council in Geneva, called on the international community to take "urgent and effective measures" to protect Syrian civilians from further attack.
Since March, she said, more than 4,000 people, including 307 children, were killed.
Meanwhile, attacks by the so-called Free Syrian Army on members of state security forces are escalating as the number of military defectors mount.
"The Syrian authorities' continual ruthless repression, if not stopped now, can drive the country into a full-fledged civil war," warned Pillay.
Peter Gooderham, the British ambassador to the United Nations, said the latest reports coming out of Syria were "harrowing."
"No one can fail to be moved by the testimony that a 2-year-old girl was shot by an officer who 'did not want her to grow into a demonstrator'," he said in a statement.
The Arab League, the Organization of the Islamic Conference and other multilateral organizations have condemned Damascus for the violence. The U.N. Security Council, however, has been unable to maneuver around Chinese and Russian objections to a formal resolution on Syria.
Syria, for its part, maintains its dealing with an armed insurgency and blames foreign elements for at least some of the bloodshed. Pillay, however, said the evidence coming out of Syria suggests the government is aware of the "consistent pattern of attacks by military and security forces on civilians."
In August, Pillay called on the Security Council to refer Damascus to the International Criminal Court.