Days before the Friday scheduled peace deal, separatist rebels set up an ambush and killed at least 9 soldiers and wounded 14 others near Timbuktu in Mali, Mali's Defense Ministry told Reuters.
A spokesman for the rebel umbrella group, the Coordination of Movements for Azawad (CMA), told Reuters they had killed about 20 people in the attack and had seized several vehicles.
The attack took place on Monday between Timbuktu and Goundam provinces where separatist groups remain strong despite the presence of French and UN troops.
The northern town of Menaka was captured by armed groups supported by the government two weeks ago, and since then rebel groups have launched a counterattack after claiming the ceasefire had been broken.
Algeria has hosted months of talks between rebel groups and Mali government but rebel groups refused to sign a peace proposes which was supposed be finalised on Friday.
The violence threatens a peace proposal which was meant to be signed on May 15 between Mali's armed groups, separatists and the government.
The UN's special representative in Mali, Mongi Hamdi, has condemned the attack.
"The violence must end immediately and I call on all parties to show commitment on the ground to the process that will result in a signing of a peace deal on May 15," Mongi Hamdi, head of the UN mission in Mali, said.
Tuareg rebel groups have been fighting against Malian forces and pro-government militias since April in the north of the country. Last week ten militants and one soldier were killed during a battle in the central town of Tenekou.