The Burkina Faso military said 24 soldiers were killed in an attack by unidentified militants on an army unit on Monday, the heaviest loss for the army in its fight to contain Islamist violence.

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. Seven other soldiers were wounded and five are still missing, it said in a statement on Tuesday.

Once a pocket of calm in the Sahel, Burkina has suffered a spillover of Islamist violence from its neighbors, including the kind of ethnic attacks that have destabilized Mali in recent years.

Large swathes of the country’s north are now out of control, raising pressure on President .

The country’s main opposition party, the Union for Progress and Change (UPC), called for the government to step down, accusing it of failing to counter a rising jihadist 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.