Bamako - Separatist rebels in northern Mali attacked positions of a pro-government militia on Monday in fighting that is undermining government attempts to pacify the region, sources from both groups said.
The Tuareg-dominated Coordination of Azawad Movements (CMA) signed a peace agreement in June that Western countries including France hoped would allow the Malian army to focus on stamping out Islamist groups, some of them linked to al Qaeda.
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But tensions have been rising again, and neighbouring Niger is due to host talks from Wednesday to ease the situation.
A spokesman for the Platform, an alliance of pro-government groups, said CMA separatists attacked two Platform positions in the early morning, the third consecutive day of clashes.
"I think that they are looking for a pretext to boycott the meeting in Niamey by saying there are clashes in the north," said Fahad Ag Almahamoud, a leader of the Gatia militia, which belongs to the Platform.
He said his group had suffered no casualties in the fighting that occurred on two fronts near the towns of Anefis and Amassine in the northern Kidal region from around 6 a.m.
Ag Almahamoud said Platform fighters seized Anefis just before midday, and a CMA combatant confirmed that the rebels had lost the town.
Mali's U.N. peacekeeping mission MINUSMA threatened to apply sanctions against those responsible for the recent violence.
"The growing number of (ceasefire) violations deeply worries the international community and risks hampering advances towards a stable and durable peace," it said in a statement.
Heavily armed CMA fighters aboard pick-up trucks raced out of the town of Kidal, the separatists' main stronghold, in the direction of the fighting early on Monday, a Reuters witness said.
CMA spokesman Almou Ag Mohamed said the rebels had launched Monday's attacks but blamed MINUSMA for failing to prevent Platform fighters making incursions into CMA-held areas.
"I don't see any point to talks in Niger or anywhere else before we are able to calm the situation on the ground."
Mali is seeking to break a decades-long cycle of Tuareg uprisings.
Most recently in 2012, rebels formed an alliance with Islamist militant groups and seized the desert north, prompting a French-led intervention that scattered the Islamists but failed to eradicate them.
Now, Islamist violence is once more on the rise and expanding further south, putting more pressure on the government to defuse tension with the Tuaregs.