Nigerian security forces have launched an operation to rescue 317 schoolgirls kidnapped by armed militants in the country’s north-west, a region at the heart of a violent banditry crisis that kills thousands each year and afflicts every corner of Africa’s most populous country.

The students were kidnapped from the Government Girls Science Secondary School in Jangebe, a remote village in Zamfara state, at about 1am on Friday. It is the largest mass kidnapping since hundreds of schoolboys were kidnapped from a school in neighbouring Katsina state in December, but comes just days after dozens of students were abducted in Niger state.

“We are angered and saddened by yet another brutal attack on schoolchildren in Nigeria,” said Peter Hawkins, Unicef’s Nigeria representative.

“Bandit” is a catchall term for roving gangs of motorcycle riders armed with bush guns and AK-47s — and, as seen in pictures following recent attacks, sometimes RPG launchers — who massacre villages and kidnap busloads of people. They have rendered major highways completely unsafe for travel and have abducted or killed people in every walk of life.

The crisis has undermined President Muhammadu Buhari’s national security credentials. The ex-general, who ruled Nigeria as a military ruler in the 1980s, was elected in 2015 on a pledge to defeat the jihadist group Boko Haram and make Nigeria safer.

But nearly two years into his second term, banditry affects the entire country, from the rugged north-west to the oil rich Niger Delta region in the south. It has surpassed the Boko Haram insurgency in the north-east, now in its 11th year, as Nigeria’s most urgent security concern.

There is no official tally of how many bandits or bandit groups operate in Nigeria. But they are thought to be a mix of organised criminal gangs and members of communities who say they have been abandoned by the state and have since turned to crime.

Some northern governors have attempted to broker peace with groups terrorising their states, drawing criticism for ostensibly offering to address the grievances of people who are killing and kidnapping their citizens, including pledges to build clinics and schools. But all of those attempts have failed.

The governor of Zamfara, Bello Matawalle, has recently joined calls to grant amnesty to those bandits who “repent” and pledge to put down their arms. But Mr Buhari has ruled out that approach, which Nigeria has applied to militants in the Niger Delta.

“Criminals are criminals and should be dealt with accordingly,” he said at a forum with northern governors on Thursday. He also warned against “ethnic profiling”. He did not elaborate, but ethnically Fulani herdsmen have been blamed for the violence.

The kidnapping in Jangebe echoes Nigeria’s most famous abduction, when Boko Haram militants took 276 schoolgirls from Chibok town in Borno state. Many have been rescued, while some were freed in exchange for millions of dollars in ransom. Roughly 100 remain missing.