Syria envoy Kofi Annan has warned that if nations trying to end the country's violence fail to act they face an international crisis of "grave severity."

He said history "will judge us all harshly if we prove incapable of taking the right path today."

The talks hosted by the United Nations at its European headquarters in Geneva on Saturday, June 30, are seen as a last-ditch attempt to salvage Annan's peace plan. Russia's determination to preserve its last remaining ally in the Middle East has collided head-on with U.S. and other Western powers' desire to replace Assad with a democracy. Diplomats are growing pessimistic that the UN-brokered conference can bridge the Russia-U.S. divide.

"By being here today, you suggest intention to show leadership, but can you follow through?" Annan said, according to the Associated Press.