Fresh fighting has broken out between rebels and government forces in the Burundian capital, Bujumbura.

The fighting has displaced thousands of civilians

A correspondent for the BBC in Bujumbura says heavy gunfire and explosions had been heard in the northern part of the capital since the early hours of Sunday morning.

"I can hear the rebels singing gospel songs near my house and bullets flying everywhere, one Western aid worker told the Associated Press news agency.

Local people have blamed the attacks on the rebel group, the Forces for National Liberation (FNL), which has been fighting since 1993 to end the political domination of the ethnic Tutsis.

On Friday, the United Nations said at least 170 people had been killed since the FLN rebels renewed their assault on Bujumbura earlier in the week.

There are scores of wounded, and more than 15,000 have been forced to flee their homes, National Security Minister Salvator Ntihabose said on Saturday.

The ethnic Hutu rebels have been demanding the resignation of the ethnically-mixed government - including President Domitien Ndayiszeye, a Hutu who took over the presidency from a Tutsi in April as part of a peace plan.

The Tutsi-led army says it has the Hutu rebels on the run

The assault on Bujumbura during the past week has been the fiercest for years.

The UN Security Council issued a statement on Friday expressing deep concern and calling for an unconditional and immediate end to the attacks.

The FNL is the smaller but more hardline of the two rebel groups opposing the government.

It has refused to take part in peace talks, or to join a ceasefire signed in December 2002 by the main Hutu rebel group, the Forces for the Defence of Democracy (FDD), and the government.

A 3,000-strong force of African Union (AU) peacekeepers is in Burundi as part of AU efforts to end the conflict, which has killed an estimated 300,000 people.