No fewer 25 Malian soldiers have been killed and scores are missing after fighters on board heavily-armed vehicles raided two army camps in the central Mopti region, according to a government spokesman.
Yaya Sangare announced the toll in a statement late on Tuesday.
"Four soldiers were wounded, around 60 are missing and there were heavy equipment losses," he said.
The raids began on Sunday evening, targeting army camps in Boulkessi and Mondoro, near the border with Burkina Faso. The bases are home to a Malian battalion of the regional G5 Sahel Force.
Sangare said the Malian army has launched an operation "to neutralise the attackers", along with forces from neighbouring Burkina Faso, backed up by French troops stationed in the region.
The army was able to reoccupy the camp in Boulkessi, and at least 15 fighters were killed and five of their vehicles destroyed, he added.
A Malian military source told the AFP news agency that a hunt was under way for the missing troops.
"Operations to secure the area are under way with Mali's partners," the unnamed source said. "Our objective is to consolidate our presence in Boulkessi and to focus on soldiers of whom we are currently without news."
The losses are a big blow to Mali's army, which is struggling to contain a revolt from groups with links to al-Qaeda and the Islamic State of Iraq and the Levant (ISIL or ISIS) group.
Mali, in West Africa, has been in conflict since 2012 when armed groups hijacked an ethnic uprising by Tuareg people in the north. More recently, the violence has moved to the centre of the country, where tit-for-tat violence between farmers and herders has also surged this year.
On March 17, the Malian army lost nearly 30 men in an attack on a camp in Dioura, also in the troubled central region.
A multi-national military force in the Sahel region, backed by France, began operations in 2017 in a bid to drive back the armed groups. The G5 Sahel Force pulls troops from Mali, Niger, Chad, Burkina Faso and Mauritania.
But a lack of finance, training and equipment, has limited the effectiveness of the joint taskforce.
Last month, West African regional group ECOWAS announced a $1b plan to counter the rising insecurity following a meeting in Burkina Faso.
At the start of the Ouagadougou summit, Jean-Claude Brou, the president of the ECOWAS Commission, pointed to the mounting human, economic and political toll of the violence and called on the United Nations to strengthen its MINUSMA peacekeeping mission, which has been based in Mali since 2013.
He said: "2,200 attacks in the last four years, 11,500 dead, thousands wounded ... millions of displaced and economic activity has been greatly affected."
Burkino Faso's President Roch Marc Christian Kabore argued that "threats transcend borders. No country is safe" and that "the escalation of violence has led to an unprecedented humanitarian crisis" in the Sahel.