The peace plan includes a cease-fire by Syrian forces, a daily two-hour halt in fighting to evacuate the injured and provide humanitarian aid and inclusive talks about a political solution.
The acceptance of Annan's proposal came in a letter on Tuesday. But there was no immediate end to the bloodshed as intense clashes between Syrian troops and rebel fighters spilled across the border into Lebanon.
The U.N. says more than 8,000 people have been killed in Syria's yearlong uprising.
The Associated Press contributed to this report.
