Twenty-four soldiers have been killed in an attack by militants in eastern Mali, the country's army has said.
Another 29 were also injured during a joint operation between troops from Mali and Niger in the Gao border region.
Mali's army said 17 "terrorists" were killed and 100 suspects were being held in Tiloa, in Niger.
It comes weeks after 54 troops were killed in another attack - one of the deadliest in the past decade.
Details of Monday's attack are unclear, and authorities have not identified what group the assailants belonged to.
Security forces have been struggling to contain the threat from Islamist groups, some of which are linked to the so-called Islamic State or al-Qaeda. In recent months, Mali's military has abandoned several outposts.
Islamist violence has also spread to other countries in the region, fueled by ethnic tensions.
According to the UN, more than 1,500 civilians have been killed in Mali and Burkina Faso since January, with millions more in need of humanitarian assistance.
A 15,00-strong UN peacekeeping force is currently stationed in Mali.