Armed men broke into a boarding school in northwestern Nigeria early Saturday and seized 15 children as they slept, police told The Associated Press, about 48 hours after nearly 300 students were taken hostage in the conflict-hit region.

School abductions are common in Nigeria’s northern region, especially since the 2014 kidnapping of more than 200 schoolgirls by Islamic extremists in Borno state’s Chibok village shocked the world. Armed gangs have since targeted schools for kidnap ransoms, resulting in at least 1,400 abducted since then.

The gunmen in the latest attack invaded the Gidan Bakuso village of the Gada council area in Sokoto state at about 1 a.m. local time, police said. They headed to the Islamic school where they seized the children from their hostel before security forces could arrive, Sokoto police spokesman Ahmad Rufa’i told the AP.

One woman was also abducted from the village, Rufa’i said, adding that a police tactical squad was deployed to search for the students.

The inaccessible roads in the area, however, challenged the rescue operation, he said, adding: “It is a remote village (and) vehicles cannot go there; they (the police squad) had to use motorcycles to the village.”

Saturday’s attack was the third mass kidnapping in northern Nigeria since late last week, when more than 200 people, mostly women and children, were abducted by suspected extremists in Borno state. On Thursday, 287 students were also taken hostage from a government primary and secondary school in Kaduna state.

The attacks highlight once again a security crisis that has plagued Africa’s most populous country. Kidnappings for ransoms have become lucrative across Nigeria’s northern region, where dozens of armed gangs operate.

No group claimed responsibility for any of the abductions. While Islamic extremists who are waging an insurgency in northeastern Nigeria are suspected of carrying out the kidnapping in Borno state, locals blamed the school kidnappings on herders who had been in conflict with their host communities before taking up arms.

Nigeria’s Vice President Kashim Shettima, meanwhile, met with authorities and some parents of the abducted students in Kaduna state on Saturday and assured them of efforts by security forces to find the children and rescue them.