Mali's Tuareg rebels clashed overnight with their former Islamist allies, witnesses said Friday, after the two groups fell out over forming a breakaway state in the northern desert region they control.

The clash involving automatic weapons near the remote regional capital Kidal was the first armed confrontation between the rebel National Liberation Movement of Azawad (MNLA) and the Islamist Ansar Dine.

The two groups are made up of Tuareg tribesmen from rival clans, and the fighting has raised fears of widening chaos in the vast northern swathe of the country, a desert region the size of France.

African leaders on Thursday urged UN backing for military intervention in northern Mali to return the region to central control.

The rebel forces faced little resistance from the Malian army when they seized control of the north while a successful military coup was unfolding in the southern capital Bamako in March.

The rival groups -- from the Taghat Melet and Idnane tribes for the MNLA and Ifora for Ansar Dine (Defenders of Faith) -- hold separate ideologies and objectives, observers say.

"Divisions are appearing within the armed Tuareg rebel groups," said Malian journalist Tiegoum Boubeye Maiga.

"The crisis is becoming tribal. After having fought the Malian army together ... the two groups are now fighting on a tribal basis. It's very dangerous."

Calm had returned by dawn Friday, an official said, but he noted that several MNLA flags had been removed from around the city.

The number of casualties was unknown.

The MNLA, calling itself "resolutely secular," said last Friday it had rejected a deal with Ansar Dine to form a breakaway state in northern Mali called Azawad because of the other group's insistence on implementing sharia, or radical Islamic law.

An Ansar Dine fighter, Mohamed Ag Mamoud, said by satellite phone late Thursday that the clash broke out because "the MNLA was manipulating civilians in Kidal to demonstrate" against Ansar Dine, which is believed to be backed by Al-Qaeda's north African branch.

"They encouraged women and children to demonstrate against us. Now we will show them our strength," Mamoud said.

Tuesday and Wednesday saw demonstrations by Kidal residents, mainly women and children, against the presence of Islamists and in support of the MNLA. The Tuesday protest was violently dispersed.

Moussa Salam of the MNLA, for his part, said: "We have been attacked, we will respond."

However an MNLA spokesman based in Paris, Mossa Ag Attaher, denied any fighting had taken place. "There were no clashes last night between the MNLA and Ansar Dine in Kidal. I spoke this morning with the MNLA executive committee ... which asserted that they were no exchanges of fire."

He added however: "We know that Ansar Dine is regrouping around Kidal, where they want to set up their headquarters."

Meanwhile on Thursday residents of Timbuktu said they had launched an armed group to kick out the Islamists currently controlling the ancient city in the far north of Mali.

The Patriots' Resistance Movement for the Liberation of Timbuktu opposes the secession of northern Mali and "will engage in military action against the invaders until they leave," said Hamidou Maiga, an ex-army lieutenant.

Timbuktu, home to priceless ancient Muslim scriptures and mosques, fell to the rebels after the coup.

UN, AU and ECOWAS officials meeting in Abidjan also called Thursday for the immediate dissolution of the former junta which came to power following the March 22 coup.

The putschists have officially ceded power to interim authorities but remain omnipresent.