A Niger government source says a joint force of Tuareg rebels began attacking the besieged northern city of Kidal using shells, rockets and gunfire.
The source, speaking to both sides in the conflict between the military and Tuaregs in Mali, says the decision to attack Thursday came after more than a week of negotiations failed.
Tuareg rebels were trying to negotiate with a Tuareg colonel commanding the Kidal garrison to surrender without a fight or to join the rebels.
Kidal would be a major prize for the rebels, who relaunched their decades-old fight in mid-January, led by battle-hardened officers and troops who fought for Libyan dictator Moammar Gadhafi and returned home heavily armed. Kidal is one of two major northern towns that failed to fall in two previous Tuareg rebellions in the 1990s and 2000s.