Unidentified gunmen seized 317 schoolgirls in northwest Nigeria on Friday, police said, the second such kidnapping in little over a week in a region increasingly targeted by militants.

School kidnappings, first practiced by jihadist groups Boko Haram and Islamic State West Africa Province, have become endemic around the increasingly lawless north, to the anguish of families and frustration of Nigeria’s government and armed forces.

Police in Zamfara state, where the latest attack took place, said they had begun search-and-rescue operations with the army to find the “armed bandits” who took the girls at Government Girls Science Secondary School in the town of Jangebe.

Zamfara’s information commissioner, Sulaiman Tanau Anka, told Reuters the assailants came in firing sporadically during the 1 a.m. raid. “Information available to me said they came with vehicles and moved the students, they also moved some on foot,” he said.

It was the third such kidnapping since December.

The rise in abductions is fuelled in part by sizeable government payoffs in exchange for child hostages, catalysing a broader breakdown of security in the north, officials have said, speaking on condition of anonymity.

The government regularly denies such payouts.

School abductions were first the domain of Boko Haram and Islamic State West Africa Province in the northeast, but the tactic has now been adopted by other militants in the northwest, whose agenda is unclear.

There was no immediate claim of responsibility for Friday’s raid.

Military shakeup

President Muhammadu Buhari replaced his long-standing military chiefs earlier this month amid the worsening violence. Armed forces in the northeast are fighting to reclaim towns overrun by insurgents.

Last week, unidentified gunmen kidnapped 42 people including 27 students, and killed one pupil, in an overnight attack on a boarding school in the north-central state of Niger.

The hostages are yet to be released.

In December, dozens of gunmen abducted 344 schoolboys from the town of Kankara in northwest Katsina state. They were freed after six days but the government denied a ransom had been paid.

Islamic State’s West Africa branch in 2018 kidnapped more than 100 schoolgirls from the town of Dapchi in northeast Nigeria, all but one of whom - the only Christian - were released.

A ransom was paid, according to the United Nations.

Perhaps the most notorious kidnapping in recent years was when Boko Haram militants abducted 276 schoolgirls from the town of Chibok in Borno state in April 2014. The incident drew widespread global attention.

Many have been found or rescued by the army, or freed in negotiations between the government and Boko Haram, also for a hefty ransom, according to sources.

But 100 are still missing, either remaining with Boko Haram or dead, security officials say.