The United Nations' chief peacekeeper vowed Tuesday to hunt down the killers of nine soldiers in the deadliest ever attack on its mission in Mali.

The soldiers, all from Niger, were on a supply run in the north-eastern desert on Friday when they were targeted by armed men on motorbikes.

Suspected Islamists on motorbikes killed nine United Nations peacekeepers from Niger on Friday in northeastern Mali, in the deadliest ever attack on the mission, military sources said.

MINUSMA did not immediately hold any armed group responsible, but a Nigerien officer from the mission told Agence France-Presse the ambush had been carried out by the Movement for Oneness and Jihad in West Africa (MUJAO), an al-Qaida-linked militia behind numerous attacks in the restive west African nation.

Five United Nations peacekeepers from Chad were killed Thursday when their truck drove over a mine in Mali, casting a shadow over peace talks between the west African state and rebel militias.

The attacks follow the restart of negotiations between the Mali government and six armed rebel groups in the Algerian capital aimed at clinching a lasting peace agreement in the deeply-divided nation.

A U.N. peacekeeper from Chad was killed and four others injured when their vehicle struck a landmine in northern Mali on Sunday, officials said.

The killing brought swift condemnation from United Nations Secretary General Ban Ki-moon and in a UN Security Council declaration.

A suspected lieutenant of al-Qaida-linked Algerian jihadist commander Mokhtar Belmokhtar has been arrested in northern Mali, security sources said on Friday.

Rebel groups in the northeastern separatist stronghold of Kidal arrested Meherig Jafar before handing him over to the authorities via the French army, a source from the United Nations peacekeeping mission told Agence France-Presse.

Four United Nations peacekeepers were killed Tuesday when their truck was blown up in Mali, security sources told Agence France-Presse, casting a shadow over the restart of peace talks between the west African state and rebel militias. 

The incident comes a day after negotiations began between the government and six armed rebel groups in the Algerian capital aimed at clinching a lasting peace agreement in the restive, deeply-divided nation.

Peace talks between the Malian government and armed rebels opened on Monday in the Algerian capital, the second round of negotiations since July aimed at clinching a lasting peace agreement.

The Bamako government and six rebel groups, mostly Tuareg but also including Arab organizations, are seeking to resolve a decades-old conflict that created a power vacuum in the desert north that was exploited by al-Qaida.

Malian government negotiators will sit down with separatist militias on Monday, hoping to end the conflicts that continue to rage in the country a year after it returned to democracy.

Riven by ethnic rivalries, a Tuareg rebellion and an Islamist insurgency in its vast desert north, the west African nation has struggled for stability and peace since a military coup in 2012.

Two Algerian diplomats kidnapped in Mali in April 2012 by the Movement for Oneness and Jihad in West Africa (MUJAO) were freed on Saturday, the Algerian foreign ministry announced.

A third diplomat, consul Boualem Sayes, had died in captivity "from a chronic illness", the ministry said.

Suspected armed Islamist extremists on Friday fired rockets at a camp of the U.N. mission in Mali in the north of the country, a Malian security source said.

The attackers fired four rockets at the MINUSMA base close to the border with Algeria at about 4:00 am (0400 GMT), the source told Agence France Presse, but could not say whether the camp at Aguelhoc had been struck or if there were any injuries.