Nigerian authorities have closed schools across some of its states after last Friday’s kidnapping of 300 schoolboys in Katsina State.

According to a report by the BBC, the states Kano, Kaduna, Zamfara and Jigawa have followed Katsina in closing schools following Friday’s attack.

The BBC reports that the Nigerian Union of Teachers has threatened a nationwide strike unless the government urgently improves the security situation.

Hundreds of students are still missing in Nigeria after gunmen attacked a boys-only secondary school in Katsina State last Friday, local and international media reported.

In the latest incident, Britain’s Guardian newspaper quoted government officials as saying a large group of men armed with AK-47s swooped on the Government Science school in Kankara on Friday night, shooting local security officers.

The union said pupils and teachers were now being actively targeted by gunmen and kidnappers.

On Tuesday, the United Nations’ secretary-general called for the “immediate and unconditional” release of hundreds of boys who were abducted during an attack on their school in north-west Nigeria late on Friday evening.

Secretary-General António Guterres strongly condemned the attack and called for the children’s safe return to their families.

Guterres said that attacks on schools and other educational facilities constituted a grave violation of human rights and urged the Nigerian authorities to bring those responsible for this act to justice.

According to international media, the Islamist militant group Boko Haram has said it was behind the raid.