BEIRUT, Lebanon — A top United Nations official urged the international community on Friday to protect Syrian civilians from a brutal military crackdown by the government of President Bashar al-Assad, as activists said that at least 11 people were shot dead in demonstrations nationwide.

In the strongest comments yet by the official, Navi Pillay, the United Nations high commissioner for human rights, said that the crackdown on pro-democracy activists seeking the ouster of Mr. Assad could lead to civil war if the world did not intervene.

“The onus is on all members of the international community to take protective action in a collective manner, before the continual ruthless repression and killings drive the country into a full-blown civil war,” Ms. Pillay said in a statement.

She stopped short of calling for military intervention, and her spokesman, Rupert Colville, said that it was for the international community to decide on the appropriate measures. World powers intervened militarily in Libya when government forces turned on protesters.