ABUJA, Nigeria (AP) — Gunmen raided an orphanage in north-central Nigeria and abducted 23 pupils, authorities said Monday. Fifteen have since been rescued.

The attack took place in an “isolated area” of Lokoja, capital of Kogi State, according to a statement by the state’s commissioner, Kingsley Femi Fanwo. The facility, Dahallukitab Group of Schools, was operating illegally, he said.

No group immediately claimed responsibility for the attack. The region has seen an increase in kidnappings for ransom.

The statement did not say how old the abducted children are, but the term “pupil” in Nigeria usually refers to someone in kindergarten or primary school, covering ages up to 12.

“Intensive operations are ongoing to secure the safe return of the remaining eight victims and apprehend the perpetrators,” Fanwo said.

Students’ kidnappings have come to define the insecurity in Africa’s most populous nation. Analysts say armed gangs see schools and students as “strategic” targets to draw attention.

Nigeria is battling a complex security crisis, especially in the north, where an insurgency has simmered for more than a decade.

Among the most prominent Islamic militant groups are Boko Haram and its breakaway faction, known as ISWAP. There is also the IS-linked Lakurawa group operating in communities in the northwestern part of the country bordering Niger.

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This version corrects the spelling to Fanwo on second reference.