BEIRUT, Lebanon — The United Nations high commissioner for human rights on Friday called for international intervention to protect Syrian civilians from the government’s crackdown, amid warnings that the country is headed toward civil war.

The commissioner, Navi Pillay, estimated that more than 4,000 people, including 307 children, have been killed in the nearly nine months since the uprising erupted against the government of President Bashar al-Assad. Ms. Pillay, who has emerged as a forceful voice on Syria, estimated that at least 14,000 people have been detained.

“The Syrian authorities’ continual ruthless repression, if not stopped now, can drive the country into a full-fledged civil war,” Ms. Pillay told an emergency session of the United Nations Human Rights Council in Geneva.