Jihadist group Boko Haram on Tuesday claimed responsibility .
"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 underway.
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.
People gathered at the Government Science Secondary School in Kankara, Nigeria, Saturday, Dec 12, 2020. Source: AP
Tuesday's claim of responsibility marks a major turning point in the advance of jihadist groups in northwest Nigeria.
Boko Haram, and a splinter group the Islamic State in West Africa Province (ISWAP), are waging an insurgency in Nigeria northeast and are thought to have only a minor presence in the northwest.
But concerns have grown of jihadist inroads into the region, especially after fighters claiming to be in the northwest released a 2020 propaganda video pledging allegiance to Boko Haram's leader.
Mr Buhari has made the fight against the group a priority, but the security situation in northern Nigeria has deteriorated since his 2015 election.
#BringBackOurBoys has been trending on social media since the weekend - a reference to a similar hashtag used after the girls' 2014 abduction.
