The spokesman, Kenneth Fiddler, told the BBC there was no indication the F-15E Eagle had been brought down by hostile fire.

Both crew members ejected safely after what was believed to be a mechanical failure, US officials say.

The Daily Telegraph newspaper says the plane went down near the rebel stronghold of Benghazi. It follows a third night of allied air strikes.

The BBC's Allan Little in Tripoli says the sky above the capital lit up with anti-aircraft fire again.

Our correspondent heard one loud explosion nearby and several distant rumbles much further afield. AFP news agency reported that a blast was heard near Col Gaddafi's Bab al-Aziziya compound.

Libyan state television reported that the capital was "under crusader enemy aerial bombardment" and that several sites had been attacked.

The Libyan authorities said a naval base at Bussetta, about 10km (six miles) east of Tripoli, and a fishing village were also hit by air strikes.

Libyan government spokesman Moussa Ibrahim said Monday's air and missile strikes had caused "numerous" civilian casualties, especially at the "civilian airport" in Sirte.

Fighting between Col Gaddafi's forces and the rebels has also continued, despite the declaration of a ceasefire by the government.

A doctor in Misrata - the last rebel-held city in western Libya - told the BBC that residents had suffered another night of shelling by government forces. "Our clinic is full of patients," he said.

There are also reports of continued fighting in Zintan, near the Tunisian border.

In the east, troops loyal to the Libyan leader opened fire with tanks, beating back an advance by opposition fighters outside Ajdabiya.