(Recasts with rebel and U.N. quotes, previous NAIROBI)

By Joe Bavier

KINSHASA, Dec 21 (Reuters) - Congolese Tutsi rebels threatened on Sunday to advance into U.N.-monitored buffer zones in eastern Congo after refusing to sign a declaration ending hostilities with the government, the rebels and mediators said.

Following several days of U.N.-backed talks in Nairobi, Kenya, the rebels led by renegade General Laurent Nkunda also declined to recommit to their own unilaterally declared ceasefire in Democratic Republic of Congo's North Kivu province.

This raised fears of a collapse of a fragile truce and of a renewal of fighting which had already driven more than a quarter of a million civilians from their homes since late August, triggering a humanitarian emergency in North Kivu.

Nkunda's National Congress for the Defence of the People (CNDP) accused government soldiers and their allies, which include Rwandan Hutu rebels, of sending troops and militia into disengagement zones intended to separate the belligerents.

"This is not behaviour that we will tolerate for very long," Bertrand Bisimwa, a spokesman for Nkunda, told Reuters.

"If these zones are not respected, we have the right to go back on our decision. We will be obliged to retake them to secure them," he added.

U.N. peacekeepers in Congo said they had detected no Congolese army movements into the buffer zones.

After launching their offensive in late August, Nkunda's battle-hardened fighters routed President Joseph Kabila's army and captured swathes of territory in North Kivu before declaring a unilateral ceasefire in late October. This ended major battles with government forces but the CNDP continued to skirmish with pro-government Mai-Mai militia and Rwandan Hutu rebels.

The talks in Nairobi aimed at cementing the ceasefire and forging a lasting peace appeared to be floundering.

"The CNDP refused to sign a joint declaration of cessation of hostilities with the government of the DRC," U.N. and African Union mediators said in a statement released on Sunday.

"Furthermore, the CNDP has declined to recommit itself to its own existing unilateral ceasefire declaration," they said, although Congo's government reaffirmed its own Nov. 18 truce.

The peace talks were due to resume on Jan. 7.

"RISK OF CONFRONTATION"

The mediators led by former Nigerian President Olusegun Obasanjo and former Tanzanian President Benjemin Mkapa said they had checked the rebel accusations and had found no evidence of Congolese government army incursions into the buffer zones.

"Nothing has changed. The situation remains as before. There is no (Congolese army) movement," Kevin Kennedy, a spokesman for Congo's U.N. peacekeeping mission (MONUC), which is charged with monitoring the disengagement zones, told Reuters on Sunday.

CNDP spokesman Bisimwa dismissed this as "lies".

"In some places they are within 150 metres of our positions ... There is a risk of confrontation," he said.

The United Nations fears that unless a political settlement can be reached the North Kivu conflict could escalate into a repeat of a 1998-2003 war which sucked in neighbouring states and devastated the vast, mineral-rich former Belgian colony.

Experts say longstanding ethnic enmities between Tutsis and Hutus in eastern Congo, stemming from neighbouring Rwanda's 1994 genocide of Tutsis by Hutus, have fuelled the conflict, along with battles to control lucrative minerals deposits.

The rebel negotiators in Nairobi called for representatives of the Congolese national assembly and senate to act as facilitators at the talks. While this was rejected by the Congolese government, the mediators said in their statement both sides were still committed to the talks.

The United Nations has agreed to send 3,000 troops to strengthen its peacekeeping force of 17,000 in Congo. But Secretary-General Ban Ki-moon has said it could take six months to deploy and called on the European Union to send troops.

Brussels, however, is split on whether to send a so-called bridging force. EU foreign policy chief Javier Solana has been charged with coming up with a response to the U.N. call. (Additional reporting by David Clarke in Nairobi; Editing by Pascal Fletcher) (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/)

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