A suicide bomber attacked a UN barracks in northern Mali on Wednesday, the United Nations said, killing three civilians and wounding 16 people, including several peacekeepers.

The militant was attempting to drive into a camp used by the UN's MINUSMA peacekeeping mission in Ansongo, in the northern region of Gao, when the explosives went off.

"The attack left nine injured, two seriously, among the peacekeepers from the Niger contingent. In addition, the explosion has killed at least three civilians. Seven (civilians) were also injured," MINUSMA said in a statement.

The mission did not specify if the bomber had been acting alone or if there were others in the vehicle but its head, Mongi Hamdi, condemned the "cowardly and odious" attack.  

"I am shocked that valiant peacekeepers are again being targeted, as well as innocent civilians," he was quoted as saying.

"This attack will not deter MINUSMA from its mission of restoring peace and security in Mali."

Divided into rival armed factions, plagued by drug trafficking and infiltrated by jihadist groups, Mali's desert north has struggled for stability since the west African nation gained independence in 1960.

The attack comes at a sensitive time as the United Nations is seeking to seal a peace deal for northern Mali and stem a wave of attacks that have targeted MINUSMA.

- Heavily-armed Islamists -

The country descended into chaos in 2012 when an insurgency by Tuareg rebels led to a coup in the capital Bamako.

Militants linked to Al-Qaeda then overpowered the Tuareg to seize control of Mali's northern desert.

A French-led military operation launched in January 2013 drove the extremists into the bush but the Tuareg rebels and Islamist militants remain active throughout the northeast of the country.

The Islamists have staged attacks on UN forces, with at least 35 peacekeepers killed since MINUSMA was deployed in July 2013 -- one of the highest tolls for a UN peace mission -- and more than 140 wounded.

The camp targeted on Wednesday is situated near the scene of the killing of a Red Cross worker, claimed by the Movement for Oneness and Jihad in West Africa (MUJAO), two weeks ago.

Last month a Chadian peacekeeper and two children died when militants fired more than 30 rockets at a UN barracks in the northern city of Kidal.

The attack on the UN base in Kidal came a day after heavily-armed Islamists attacked a Bamako nightclub, killing five people, including a French national and a Belgian.

Meanwhile Mali's main Tuareg rebel alliance, known as the Coordination for the Movements of Azawad (CMA), has refused to sign a peace deal agreed by other armed groups and the government.

Algeria, a mediator in the peace process, had scheduled a ceremony for Wednesday during which the Coordination's representatives were expected to initial the accord.

- Summary executions -

Human Rights Watch (HRW) called on the government this week to curtail abuses by armed groups and state security forces, describing  "widespread lawlessness and insecurity" across the north.

"Rampant criminality and attacks by armed groups and abuses by the security forces are putting ordinary people in central and northern Mali at risk," Corinne Dufka, the group's west Africa director, said in statement released on Tuesday.

"Armed groups need to stop their abuses and Mali's government should take urgent steps to reverse this trend, which threatens the security and rule-of-law gains of the past two years."

The statement said MINUSMA had been the target of at least 79 attacks since its inception.

HRW said it had conducted interviews in February and March with more than 150 victims and witnesses of abuses in Gao and in the capital, Bamako, including shepherds, prisoners, government and UN officials and community leaders.

It said it had found that, in central Mali, an Islamist armed group referred to as the Macina Liberation Movement had recently committed serious abuses in the course of military operations against security forces.

The attackers had summarily executed at least five men believed to have worked as guides or to have provided information for the army.

"In public meetings and flyers distributed in towns and villages, the group threatened the local population with death if they collaborated with French forces, the government, or the UN peacekeeping mission," HRW said.

Witnesses described the use of child soldiers, some as young as 12, by rebel groups, it added.