By Joe Bavier

UVIRA, Congo, Aug 24 (Reuters) - Unrest among former-Tutsi rebels integrated into Democratic Republic of Congo's army threatens to shatter an 8-month ceasefire and plunge the country's troubled east back into war.

On Thursday, troops loyal to renegade General Laurent Nkunda, who led a 2004 uprising in the volatile North Kivu province to defend his Tutsi people, began abandoning their positions without the blessing of military top brass.

The troop movements have sown confusion over the fate of five mixed brigades involving Nkunda's soldiers created under a January truce brokered by neighbouring Rwanda.

It coincided with a wave of desertions by government soldiers within the brigades amid rising tensions between them and Nkunda troops. Nkunda accuses Kinshasa of abetting his enemies in the Democratic Forces for the Liberation of Rwanda (FDLR), a Hutu rebel movement based in eastern Congo.

"The mixed brigades no longer exist," one government commander, who asked not to be named, told Reuters on Friday. "(Nkunda's men) have pulled out of the mixed brigades."

Congo's military commander in North Kivu, General Vainqueur Mayala, said he did not know of any official decision by Nkunda loyalists to break the ceasefire.

"There have been battalion movements here and there since yesterday," he said. "I cannot say, yes or no, if they have withdrawn from the army. I'm going to make a visit to the ground and will know more tomorrow."

Commanders loyal to Nkunda were not reachable by telephone.

The United Nations Mission in Congo (MONUC), the world's largest with 17,000 peacekeepers, has reinforced its presence in North Kivu as tensions have grown.

"We also had these reports of troop movements in the mixed brigades. We're following them closely," said Sylvie Van Den Wildenberg, U.N. spokesperson in North Kivu. "The situation is tense."

PEACE ELUSIVE

In 2004, Nkunda led two army brigades, or around 4,000 soldiers, into the bush and briefly captured Bukavu, the capital of neighbouring South Kivu. He still faces an international arrest warrant for war crimes allegedly committed during the brief occupation.

Following elections last year aimed at drawing a line under a 1998-2003 war which killed some 4 million people, President Joseph Kabila promised to bring peace to Congo's east.

In a November crackdown, MONUC helicopters and armoured vehicles killed hundreds of Nkunda's fighters, paving the way for the January peace deal. The mixed brigades were meant to help pacify North Kivu, long a stronghold of militias, foreign and domestic rebels.

Instead, 165,000 people have fled fighting between the Tutsi-dominated brigades and the FDLR. Congo's decision this month to halt operations against the FDLR until the brigades could be replaced by regular army was met with anger by Rwanda.

In an interview this week, Nkunda accused Kabila's government of supporting the FDLR, made up of remnants of the Rwandan army and Interahamwe militia responsible for the 1994 genocide that killed some 800,000 Tutsis and moderate Hutus.

"The international community must know who is the mastermind behind this problem. I want to show the national community what Kabila is doing with those FDLR," he told Reuters. "I don't think we are going to stay very long."

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