The BBC's Ishbel Matheson: "The Lusaka peace process has inched forward."

But Jean-Pierre Bemba, leader of the Congolese Liberation Movement (MLC), threatened to resume fighting if a separate rebel group did not sign its part of the truce within a week.

"It is true I have signed [a ceasefire] today," he said on Sunday.

"But I will withdraw my signature unless the others also sign within seven days."

Mr Bemba was referring to the Rwandan-backed Congolese Rally for Democracy (RCD), which controls about half of Africa's third largest country and is in disarray over the accord.

He said that the MLC, which is backed by Uganda, would "continue the fight to Kinshasa", if the RCD did not meet his deadline.

Jean-Pierre Bamba: Truce will not hold unless RCD also signs

Earlier the millionaire Mr Bemba committed his forces to the ceasefire at a ceremony presided over by conflict mediator Zambian President Frederick Chiluba.

"We believe the region needs real peace," he told a news conference after the signing.

"It is for this reason that we have signed the ceasefire to bring an end to the war."

He called on Congolese President Laurent Kabila to step down and warned that his troops would retaliate immediately if attacked by government forces.

Mr Chiluba applauded his "responsible leadership", and is expected in Rwanda on Tuesday to try to persuade the RCD to sign the deal.

The signing by Mr Bemba follows several significant victories by his troops over government forces in recent days.

Slow path to peace

The peace pact, which was reached on 10 July, is aimed at ending a complex war which started with a rebel insurgency in August last year.

It has so far been signed on the one hand by President Kabila and his military allies from Zimbabwe, Angola and Namibia, and on the other by Rwanda and Uganda.

But a dispute over leadership meant the RCD did not sign and neither did the MLC.

The former has split into two factions, one led by ousted leader Ernest Wamba dia Wamba and another by Emile Ilunga.

Ilunga, who is backed by Rwanda, says he will not sign the document along with Wamba, who is supported by Uganda.