Gunmen have killed at least 57 members of a local self-defence vigilante group in clashes in northwestern Nigeria, a security source and local residents said on Tuesday.

President Muhammadu Buhari condemned the “brutal murder of tens of vigilante members” in the latest violence involving heavily armed gangs known locally as bandits, who raid and loot villages across northwest Nigeria.

Police confirmed Monday’s clashes in Zuru district of Kebbi State, without giving a casualty figure. But the security source said 57 bodies had been recovered while two local residents said 62 people were killed.

Local residents often form informal vigilante units, known as Yansakai, to protect villages from bandit raids, though some states banned them after they were accused of abuses and extra-judical killings.

“There was an incident involving Yansakai and bandits which left several dead on both sides,” Nafiu Abubakar, Kebbi state police spokesman, told AFP.

The bandits were fleeing ongoing military operations on their enclaves in neighbouring Niger state when they were intercepted by the vigilantes, who had been tipped off about their movements, Abubakar said.

Local resident Almu Sallami said the vigilantes had mobilised from different villages to take on a large convoy of heavily-armed bandits.

Northwest and central Nigeria are a hub of criminal gangs which maintain camps in Rugu forest, straddling Zamfara, Katsina, Kaduna and Niger states, where they raid villages, steal cattle and carry out mass kidnappings for ransom.

“This egregious level of criminality is shocking and I want to assure Nigerians that I will do all it takes to tackle this monster decisively,” Buhari said, urging security forces to increase efforts.

Security experts have warned the gangs who are driven by financial motives are increasingly forging alliances with jihadists from the northeast waging a 12-year old Islamist insurgency.

Security forces say they have been bombarding and raiding forest hideouts while authorities last year also cut telecommunications in some northwest states in a bid to disrupt bandit communications.

In January, Nigeria’s government labelled the criminal gangs as terrorist groups in a bid to bring tougher sanctions against the gunmen and their informants.

Criminal gangs in the northwest made international headlines last year when they raided a number of schools and kidnapped students in a bid to squeeze more ransom out of communities.

Nigeria’s bandit violence has its roots in the clashes over land and resources between farmers and nomadic cattle herders in the northwest where tit-for-tat attacks have spiralled into broader criminality.