At least two Senegalese UN troops were killed Saturday in a suicide attack in Mali’s northeastern rebel bastion of Kidal, a day before a second round of legislative polls.

At least two Senegalese UN troops were killed Saturday in a suicide attack in Mali's northeastern rebel bastion of Kidal, a day before a second round of legislative polls, a UN source said.

The soldiers died when a suicide bomber ploughed his explosives-laden car into a bank that they were guarding.

The suicide bomber also died and the attack left several wounded.

A local Kidal official earlier said Senegalese soldiers were among those who guarded the BMS bank.

French President Francois Hollande said this "heinous crime cannot go unanswered", while Foreign Minister Laurent Fabius condemned what he called a "cowardly act on the eve of the second round of Mali's legislative elections".

Saturday's attack came a day before Mali was to hold a second round of legislative elections, the fourth time that the former French colony in west Africa has gone to the polls in less than six months.

The vote is the final stage in Mali's return to democracy after a March 2012 coup that threw it into turmoil and opened a power vacuum that enabled the extremists to seize the north.

An African military source in Timbuktu called the operation "very large... the largest in the Timbuktu region since allied forces retook the main northern cities".