Jihadist group Boko Haram on Tuesday claimed responsibility for the abduction of hundreds of school students in north-west Nigeria.

“I am Abubakar Shekau and our brothers are behind the kidnapping in Katsina,” said the leader of the group that was also behind the 2014 abduction of hundreds of schoolgirls in Chibok, in a voice message.

At least 333 students are still missing since the attack late Friday on the all-boys Government Science Secondary School in northwestern Katsina state.

The attack was initially blamed on armed groups locally known as “bandits”, who are active in the unstable region where kidnappings for ransom are common.

The army said Monday it had located the hideout of the men, and that a military operation was under way.

More than 100 gunmen on motorcycles stormed the rural school north of Kankara town, forcing students to flee and hide in the surrounding bush.

A number of boys were able to escape, but many were captured, split into groups and taken away, residents told AFP.

The kidnappings occurred in the home state of President Muhammadu Buhari, who condemned the attack and ordered security stepped up in schools, with those in Katsina state closed.

Tuesday’s claim of responsibility marks a major turning point in the advance of jihadist groups in northwest Nigeria.