Malian troops bolstered security at army checkpoints and villagers detained two youths allegedly strapped with explosives on Saturday after Islamists claimed responsibility for the country's first suicide attack.
Residents of a village near Gao, the largest city in the north, said they had detained two young men, an Arab and a Tuareg, whom they claimed were wearing explosive-rigged belts and travelling on the same road where the suicide bombing on Friday wounded a soldier at a checkpoint.
In Timbuktu, a grave was discovered Friday containing several bodies including those of three Arab shopkeepers who had recently been arrested by the Malian army, the independent Mauritanian news site ANI reported.
Northern Mali is being torn by rising tensions between light-skinned Arabs and Tuaregs -- often accused of supporting the Islamist occupiers who seized control of the north for 10 months -- and their black neighbours.
Rights groups have accused the Malian army of summary executions of Tuareg and Arabs and called on the government to protect them from reprisal attacks.
Troops in Gao were fortifying checkpoints with sandbags and heavy machine guns and patrolling the city in heavy rotation after Friday's suicide blast, carried out by a young Tuareg who rode a motorcycle up to a checkpoint and detonated an explosive belt.
The Movement for Oneness and Jihad in West Africa (MUJAO) claimed the attack and vowed to carry out more against "the Malian soldiers who chose the side of the miscreants, the enemies of Islam".
MUJAO is one of a trio of Islamist groups that seized northern Mali before France sent in fighter jets, attack helicopters and 4,000 troops to drive them out.
The French-led operation, launched on January 11 as the insurgents advanced toward the capital Bamako, has succeeded in forcing the Islamists from the towns under their control.
But they are thought to retain a presence in the vast desert spaces of the country's north, and France is now anxious to hand over the operation to UN peacekeepers amid fears of a prolonged insurgency.
In Bamako, heavily armed Malian soldiers surrounded a base housing rival paratroopers where a firefight the day before killed two adolescents and wounded another 13 people.
The clash between the rival units highlighted the deep divisions in the Malian military.
The paratroopers are loyal to ex-president Amadou Toumani Toure, ousted in a March 2012 coup, and were protesting an order absorbing them into other units to be sent to the frontline.
The nation imploded last year after the coup, waged by soldiers who blamed the government for the army's humiliation by a rebellion among the Tuareg, a north African people who have long complained of being marginalised by Bamako.
A month later, paratroopers launched a failed counter-coup that left 20 people dead.
With Bamako in disarray, Al-Qaeda-linked fighters hijacked the Tuareg rebellion and took control of the north, imposing a brutal form of Islamic law.
-- Fears of fallout --
Malian prosecutors issued international arrest warrants Friday for 26 Tuareg and Islamist rebel leaders on charges including terrorism, war crimes and drug trafficking.
The move was condemned by one of the groups targeted, the Islamic Movement for Azawad (MIA), which had recently broken away from Islamist group Ansar Dine (Defenders of the Faith) and offered an olive branch to Bamako by renouncing "extremism and terrorism".
"Mali is showing its determination to thwart a political solution, the approach advocated by the international community," it said.
A regional security source also warned the arrest warrants could complicate talks on Mali's future, saying: "I don't see how they're going to negotiate with people they want to arrest."
Mali's interim government has rejected negotiating with Islamist groups but has voiced willingness to talk with the Tuareg rebels in exchange for dropping their independence demands.
In Paris, prosecutors brought terrorism charges Saturday against four men arrested last week as part of an investigation into a network to send jihadists to fight in Mali.
The men have links to Cedric Lobo, a Frenchman arrested in Niger last year on his way to Timbuktu to join Islamist groups in northern Mali, a judicial source said.
Fears of fallout from the Mali crisis also surfaced in neighbouring Mauritania, where the opposition condemned President Mohamed Ould Abdel Aziz's recent expression of support for the campaign.
"The spark of this war is bound to reach Mauritania," opposition leader Ahmed Ould Daddah told reporters in the capital Nouakchott.