GENEVA — A conference called by special envoy Kofi Annan to end the Syria crisis appeared on the brink of failure as it opened Saturday, with the U.S. and Russia still divided over a role for President Bashar Assad in a transition government.

Annan seemed confident of his plan a few days ago, but Russia has refused to back a provision that would call for Assad to step down to make way for a unity government, a stance that could scuttle the entire deal.

The envoy warned the permanent members of the U.N. Security Council — Britain, China, France, Russia and the United States — that if they fail to act at the talks hosted by the United Nations at its European headquarters in Geneva they face an international crisis of "grave severity" that could spark violence across the region and provide a new front for terrorism.