President Ndayizeye has threatened to crush the Hutu rebels

The fighting started early on Monday morning and was concentrated in the southern suburbs of the capital.

The army reacted by firing back at the rebel positions.

The attack was the first sustained shelling since President Domitien Ndayizeye - himself a moderate Hutu - took office on 30 April for the second half of a three-year transitional government.

President Ndayizeye has threatened to use all necessary means to crush Hutu rebel fighters who failed to join the peace process that brought him to power.

Deserted

The FNL say the attack is a warning to the government of President Ndayizeye to enter into negotiations with them.

"We attacked Bujumbura to prove to the government and the army that we are still able to come down to the capital," Reuters news agency quoted FNL spokesman Pasteur Habimana saying.

The streets of the capital were deserted this morning as people opted to remain indoors.

The BBC's Prime Ndikumagenge says there are fears that the situation could degenerate into street fighting between the rebels and the government army.

Street fighting

The rebels are not in uniform and the ordinary residents of Bujumbura fear that they could be mistaken for rebel fighters.

There are also growing fears that if the fighting continues the capital could face shortages of essential supplies as the rebels control supply routes from the south.

Our reporter was not able to establish casualty figures.

Fighting has continued in Burundi despite a ceasefire agreement signed in December 2002 by the main Hutu rebel group, the Forces for the Defence of Democracy (FDD), and the government.

The FNL has not signed the agreement.

A peacekeeping force made up of 3,000 soldiers from African Union member-countries is in Burundi as part of efforts to end the fighting between the Tutsi-led army and Hutu rebels.

An estimated 300,000 people have been killed in the fighting.