BBC News with Marion Marshall

The United Nations special envoy Kofi Annan has warned that the current peace plan for Syria may be the last chance to avoid civil war. He said such conflict couldn't be allowed to happen and its implications would be frightening. Mr Annan was speaking after he briefed the UN Security Council on the situation in Syria. Barbara Plett reports from New York.

The tone of Kofi Annan's comments was

bleak

. He acknowledged that the government's use of heavy weapons had declined somewhat since the ceasefire declaration, but he said the level of violence remained unacceptable. He tried to sound a note of optimism, saying the presence of the UN military observers had helped to bring calm in places, and the deployment of the full team by the end of the month would

make quite a difference

. But he suggested that the peace plan might fail, and warned that this could mean full-scale civil war. Western envoys, such as the US ambassador Susan Rice,

signal

that they do stand ready to seek tougher action at the council, like sanctions; but it's almost certain that Russia and China wouldn't support such moves.

Libyan security forces have prevented armed protesters from storming the headquarters of the prime minister in Tripoli. The attackers were former rebels from outside the capital, who fought to overthrow Colonel Gaddafi last year, and were demanding compensation promised by the government. One member of the security forces was killed. Nasir al-Mani, who's a spokesman for the prime minister, said the government needed greater protection.