View our special coverage: 'Conflict in North Kivu'

Also view our correspondent's report on the plight of child soldiers : 'Children take up arms in North Kivu'

Demoralised Congolese government troops, retreating before eastern rebels, clashed on Tuesday with their own local militia allies who tried to make them stand and fight after the armed forces chief was replaced.

Democratic Republic of Congo President Joseph Kabila late on Monday sacked and replaced his military chief of staff, General Dieudonne Kayembe, in a bid to bolster the fighting capacity of soldiers who are fleeing before the well-armed Tutsi rebels.

Government troops falling back before a rebel advance on Kanyabayonga in eastern North Kivu province clashed with their own allies of the Pareco Mai-Mai militia, whose pro-government commanders said they wanted the soldiers to halt the rebels.

"We are stopping them and trying to force them back to the front. That is their job. We don't understand how they can flee when the rebels are about to come to Kanyabayonga," Pareco Mai-Mai leader General Sikuli Lafontaine told Reuters.

"These soldiers are cowards. They just flee and then rape and pillage in the cities," he added. Local residents said they saw the bodies of soldiers and Pareco Mai-Mai militiamen killed.

There was no immediate reaction from regular army commanders. Congolese troops in the combat zone complained of not being paid and of not trusting their senior officers.

The confused fighting at Kirumba and Kayna not far from the shores of Lake Edward, which witnesses said involved machine-gun and rocket-propelled grenade fire, raged on despite intense diplomatic efforts to bring peace to eastern Congo, where weeks of combat has displaced a quarter of a million people.

Kabila's government and U.N. peacekeepers say Tutsi rebel leader Laurent Nkunda, whose fighters are waging a four-year-old rebellion in North Kivu, are not respecting a ceasefire Nkunda himself vowed to maintain in weekend talks with a U.N. envoy.

Nkunda and his commanders accuse the army of "provocation".

Kabila named as his new armed forces head former navy chief General Didier Etumba, who had previously served as leader of military intelligence, Congolese state television said.

As aid workers struggle to help hundreds of thousands of refugees in North Kivu, many of them hungry and sick, U.N. Secretary-General Ban Ki-moon has asked the Security Council to urgently reinforce the U.N. peacekeeping force in Congo.

U.N. diplomats said on Monday the Council hoped to vote this week on a French-drafted resolution that would add 3,000 extra troops and police to the 17,000-strong U.N. force in Congo, which is already the biggest of its kind in the world.

The U.N. force, known as MONUC, has been criticised by aid agencies and Congo's government for repeatedly failing to protect civilians from attacking rebels and ill-disciplined government soldiers who kill, loot and rape as they retreat.

Kabila's government and its western allies have been struggling to put together a national army from the patchwork of central army soldiers and former rebel factions that fought in Congo's 1998-2003 war, which sucked in six African states.

But old allegiances, lack of training and discipline and a tradition of seizing booty from the land have made this a difficult task, compounded by ethnic tensions in east Congo.

One Congolese government officer at Luofu near Kirumba, who asked not to be named, listed a long litany of complaints.

"Our soldiers are angry. We haven't received our money for a year .... When the money doesn't come it is like a heavy weapon against us. Our wounded are not well looked after, and we are not well fed," he said. He added the Pareco Mai-Mai militia were preventing the army from retreating into its territory.

The officer said his men did not trust their army land forces chief, General Gabriel Amisi, also known as "Tango Four", because he was a former comrade of rebel leader Nkunda from the Rwandan-backed RCD rebel group that fought in the 1998-2003 war.

"Tango Four was RCD like Laurent Nkunda. We are trying to fight, but they know each other politically," he said.

Congo's North Kivu conflict traces its origins back to Rwanda's 1994 genocide, when Hutu militias killed about 800,000 Tutsis and moderate Hutus before fleeing into eastern Congo.

Nkunda says his rebellion is protecting east Congo's Tutsi minority and accuses Kabila of using a Rwandan Hutu rebel group, the FDLR, which includes perpetrators of the 1994 genocide, to fight against him. Congo accuses neighbouring Rwanda of supporting Nkunda's rebellion, a charge denied by Kigali.

The U.N. envoy who met Nkunda on Sunday, former Nigerian President Olusegun Obasanajo, said Nkunda agreed to take part in peace talks in Nairobi, but Congolese President Kabila has not confirmed he is ready to meet Nkunda face to face.

Nkunda says he wants to discuss Congo's future with Kabila.