The National Movement for the Liberation of Azawad (MNLA) has ended a ceasefire agreement with the Malian government, in force since June.
The Tuareg separatist group made the move after clashes between protestors, who demonstrated against a visit by President Oumar Tatam Ly, and Malian troops.
"What happened is a declaration of war. We will deliver this war," the MNLA's vice-president, Mahamadou Djeri Maiga, told AFP.
"Wherever we find the Malian army we will launch the assault against them. It will be automatic. The warnings are over."