"We decided on an immediate ceasefire and on an immediate stop to all military operations," Foreign Minister Moussa Koussa told reporters in Tripoli, after the UN Security Council passed a resolution authorizing military action.

He called for dialogue with all sides. Gaddafi had vowed to show "no mercy, no pity" on Thursday, and rebels pleaded for foreign aid before time ran out. They said the city of Misrata was being pounded by government forces on Friday morning.

France, a leading advocate of military action, said it was cautious about the ceasefire announcement and that the "threat on the ground has not changed."

Western officials said military action could include France, Britain, the United States and one or more Arab countries.

"Britain will deploy Tornadoes and Typhoons as well as air-to-air refueling and surveillance aircraft," Prime Minister David Cameron told parliament.

"Preparations to deploy these aircraft have already started and in the coming hours they will move to airbases from where they can start to take the necessary action."

Gulf state Qatar said it would take part but it was unclear whether that meant military help, while Denmark said it planned to contribute warplanes. France is to host international talks on Saturday to discuss the action.

People in Misrata said the rebel-held western city was under heavy bombardment by Gaddafi's forces on Friday.

"They are bombing everything, houses, mosques and even ambulances," Gemal, a rebel spokesman, told Reuters by phone from the last big rebel stronghold in the west.

Time was also running short for Benghazi, the eastern city that has been at the heart of Libya's month-old revolt.

But Gaddafi's troops did not fulfill his threat to overrun the rebel base overnight after their rapid counter-offensive brought them to within 100 km (60 miles) of the eastern city.

"We will come. House by house, room by room," Gaddafi said in a radio address to Benghazi late yesterday.

Gaddafi's son Saif al-Islam said Libya was "not afraid" of the U.N. resolution, Al Arabiya said.

He said the army would surround but not enter Benghazi and "anti-terror" forces would be sent in to disarm rebel forces, Al Jazeera quoted ABC news as saying.

Al Jazeera television showed thousands of people listening to Gaddafi's speech in a central Benghazi square, then erupting in celebration after the UN vote, waving anti-Gaddafi tricolors and chanting defiance of the man who has ruled for four decades.

Fireworks burst over the city and gunfire rang out.

Some had fled to the Egyptian border yesterday but said the UN move had given them new hope. "It's a great development. We are so thankful," said Rajab Mohammed al-Agouri, with five children. "But we are waiting for it to be implemented. We are tired of talk."