Mozambique security forces have killed at least 129 insurgents in the northern Cabo Delgado region that has been besieged by violence for at least the last three years.

The interior minister said this late Tuesday.

Since 2017, infrequent but violent raids on government buildings and villages by militias with suspected links to the Islamic State have intensified in the gas-rich northernmost province of one Africa’s poorest nations.

The interior ministry said the 129 killings were the total for the month, and were a retaliation for an attack in Xitaxi in Muidumbe district earlier in April, where insurgents killed 52 villagers.

Little is known about the insurgents, though initial attacks were claimed by a group known as Ahlu Sunnah Wa-Jama. More recently, Islamic State has claimed a number of attacks.

Security officials have struggled to contain the attacks. Since clinching re-election in January President Filipe Nyusi has vowed to dedicate more resources to fighting the insurgency.

Exxon Mobil and Total, among the multi-national oil majors developing the gas projects off the shore of northern Mozambique thought to be worth more than $60 billion, have expressed concern the violence could affect operations.

Interior minister Amade Miquidade said in a statement that 39 extremists were killed when they attempted to invade Muidumbe village on April 7, with 59 killed in a gun battle in Querimba islands three days later.

31 more were killed in Ibo Island between April 11 and April 13, the statement said.