BEIRUT - The head of the UN observers in Syria said a spike in bloodshed is derailing the mission to monitor and defuse more than a year of violence, raising questions on Friday about how effective the unarmed force can be in a conflict that every day looks more like a civil war.

The troubles facing the observer mission are the latest sign that an international peace plan for Syria is disintegrating. Western powers have pinned their hopes on the plan, brokered two months ago by special envoy Kofi Annan, in part because there are no other options on the table. There is little support for military intervention, and several rounds of sanctions have failed to stop the bloodshed.