Gunmen killed 24 soldiers in an attack on an army unit in Burkina Faso, the deadliest yet in the West African nation's fight against militants, which the president on Tuesday called a "dark stain" on the country's history.
Seven other soldiers were wounded in the attack on Monday and five are still missing, the military and President Roch Marc Kabore said in separate statements.
The army, which earlier put the death toll at 10, said it had launched a land and air operation in response to the attack in Koutougou, in northern Burkina Faso's Soum province.
"August 19th is a dark stain on the life of our national army," Kabore said in the statement posted on the presidency website. "It is a heavy toll, which ... saddens us."
Once a pocket of relative calm in the Sahel, Burkina has suffered a homegrown insurgency for the past three years, which has been amplified by a spillover of violence and criminality from its chaotic neighbour Mali.
Large swathes of Burkina's north are now out of control, and France's military Sahel mission began limited operations there earlier this year.
The main opposition party, the Union for Progress and Change (UPC), called for the government to step down, accusing it of failing to counter a militant threat which has killed hundreds of civilians and caused more than 150,000 to flee.
"It's a team overwhelmed by the turn of events, which is currently crossing its arms, waiting for how fate will play them and the Burkinabes," UPC said in a statement.
Deteriorating security prompted the Ouagadougou government to declare in December a state of emergency in several northern provinces bordering Mali, including Soum.