The violence, which began in March 2011 as a protest against Bashar Assad's regime, has claimed more than 9,000 lives, the United Nations reported. The Local Coordination Committee of Syria, an opposition group, puts that number closer to 11,000.
At least 17 people were killed Tuesday, despite an agreed-upon cease-fire implemented April 12, CNN reported. Annan, who has proposed a six-point peace plan, said there are serious violations of the cease-fire on both sides.
"Government troops and armor are still present, though in smaller formations," Annan said Tuesday. "There have been worrying episodes of violence by the government but we have also seen attacks against government forces, troops and installations. And there has been a spate of bombings, which are really worrying, and I'm sure creates incredible insecurity among the civilian population.
"I believe that the U.N. Supervision Mission is possibly the only remaining chance to stabilize the country," he added. "There is a profound concern that the country could otherwise descend into full civil war and the implications of that are quite frightening. We cannot allow that to happen."