Lavrov has recently criticized Assad for dragging his feet on reforms and using excessive force. He and other Russian officials have strongly urged their old ally to observe Annan's plan.
Russia, along with China, has twice shielded Assad's regime from U.N. sanctions over its deadly crackdown on a popular uprising. But Moscow has strongly supported Annan's cease-fire plan to end 13 months of violence and begin talks on Syria's political future.
Associated Press writer Vladimir Isachenkov in Moscow contributed to this report.