French troops killed 19 Islamist militants during an army operation in Mali's rebel-infested northern desert on Tuesday, a French military source told Agence France Presse, as the country prepared to stage nationwide elections.

The violence comes with Malians due to vote on Sunday in a second round of parliamentary polls supposed to mark the west African nation's first steps to recovery after it was plunged into chaos by a military coup in March last year.

Nigeria said on Wednesday that 500 people who were arrested during security operations against Boko Haram militants in three northeast states should be put on trial for terror offences.

The 500 are among nearly 1,400 detained in Borno, Adamawa and Yobe states between July and September, the country's defense spokesman, Chris Olukolade, said in a statement.

Twenty-one bodies, believed to be those of soldiers close to Mali's ousted president Amadou Toumani Toure, were found overnight Tuesday in a mass grave near the capital Bamako, officials told Agence France Presse.

"We have found 21 bodies, probably of 'red beret' soldiers, in a mass grave in Diago. The bodies were exhumed," a Malian justice ministry official said.

The Malian government remains "open to talks" with Tuareg rebels despite one of their leaders declaring a return to war, Prime Minister Oumar Tatam Ly said Tuesday.

Tatam Ly, in an interview with the pro-government daily L'Essor, downplayed the war declaration made Friday by Mahamadou Djeri Maiga, vice president of the Tuareg's National Movement for the Liberation of Azawad (MNLA).

Mali's defense minister vowed Monday he would not pose "any obstacle" to the court case against General Amadou Sanogo, the leader of a March 2012 coup that plunged the country into crisis.

Sanogo, a divisive figure in the west African country, was arrested on November 27 and charged along with 15 other people, mostly fellow soldiers from his inner circle, for alleged crimes during the coup and its aftermath.

A suicide bomber staged a botched attack on foreign forces in northern Mali overnight Saturday, killing himself but causing no other casualties, Malian and French military officials said.

A senior Malian official described the bomber's target as a "French army position," but a French army spokesman said the attack was against a battalion of the U.N'.s peacekeeping mission for Mali, MINUSMA, and that only 24 French troops were present.

The ethnic Tuareg rebellion announced Friday it would return to war against Mali's army after it said one person was killed and five others injured in clashes with soldiers at an airport.

"What happened (on Thursday) is a declaration of war. We will deliver this war," said Mahamadou Djeri Maiga, vice-president of the MNLA, or the National Movement for the Liberation of Azawad.

Tuareg demonstrators Thursday occupied an airport runway to prevent Mali's Prime Minister Oumar Tatam Ly visiting the rebel-controlled northeastern town of Kidal, officials said.

Protesters said Malian soldiers shot and wounded three of the demonstrators, but the Malian army denied that.

The European Union said on Monday Mali's parliamentary elections had confounded fears over possible Islamist violence and were "another success" despite low-level protests in the north and a poor turnout.

Louis Michel, head of the bloc's election observation mission, paid tribute to "the success of the organization of elections, particularly with regard to the logistical, material and human conditions that prevailed during voting operations".

Malians voted on Sunday in parliamentary elections intended to seal the troubled west African nation's return to democracy but which were marred by low-level civil unrest and apparently poor voter turnout.

The polls marked Mali's first steps to recovery after it was plunged into chaos by a military coup in March last year, and finalized a process begun with the election of its first post-conflict leader in August.