Witnesses said at least 30 civilians were killed when the rebels attacked two suburbs of Bujumbura in the early hours of Sunday. In retaliatory fighting, army soldiers said they killed 20 rebels.

But in a BBC interview a spokesman for the main rebel faction, Jerome Ndiho, denied that the rebels had killed the civilians and blamed the army.

Strong action

President Buyoya, a retired Tutsi general, condemned the killings, saying the attackers would be unable to claim that those who died were an obstacle to any political end.

The BBC's Cathy Jenkins: "The rebels have taken several hostages"

"Now is not the time for words," he told reporters after returning from a trip to South Africa.

"We need to step up our actions to really punish those genocidal terrorists," he said.

Officials said the rebels had retreated from the city, but that fighting continued in the surrounding area.

The militia were also reported to have taken several hostages.

Fierce attack

The attack was concentrated on the neighbourhoods of Musaga and Mutanga. Residents said they set a number of houses and cars ablaze, and shot at any Tutsis living there.

A reporter for Reuters news agency said he saw the bodies of many children who had been shot dead.

One report said some people had been burnt to death inside their homes.

Burundi's Defence Minister, Colonel Alfred Nkurunziza, told reporters at least 20 rebels had been killed in fighting with army troops, but that no soldiers had died.

Gunfire and mortar explosions were heard in the capital from midnight to dawn.

Burundi's Tutsi-led army has been fighting rebel Hutu groups since 1993.

Several rounds of peace talks have so far failed to end the conflict, which has killed an estimated 200,000 people. A new round of negotiations is scheduled to take place next month.

Hutus form the majority of the country's population, but the minority Tutsis have traditionally dominated the military and economic institutions,

A recent report by Amnesty International accused both the army and the rebel Front for the Defence of Democracy of killing civilians indiscriminately.