More states in northern Nigeria have ordered all schools to close following last week's kidnapping of hundreds of pupils in Katsina state.

Kano, Kaduna, Zamfara and Jigawa have followed Katsina in closing schools following Friday's attack.

The Islamist militant group Boko Haram has said it was behind the raid.

More than 300 children are still missing, raising fears for the safety of other schools, especially those in remote areas.

Meanwhile, the Nigerian Union of Teachers has threatened a nationwide strike unless the government urgently improves the security situation.

The union said pupils and teachers were now being actively targeted by gunmen and kidnappers. It said the attack was a sad reminder of previous raids - dozens of girls from Chibok, in northern Borno state, are still missing six years after they were abducted by jihadists.

Nigerian authorities say have been in contact with the kidnappers in the latest incident, but there are no details of the discussions.

The governor of Katsina state, Aminu Bello Masari, said on Twitter late on Monday: "Talks are ongoing to ensure [the pupils'] safety and return to their respective families."

The jihadist group issued its claim of responsibility in a four-minute recording.

However, security and local sources cited by AFP news agency said Boko Haram had recruited three local gangs to carry out the attack.

One source said the children had been taken across the border into Zamfara state and divided among different gangs "for safe keeping". Some of the gangs had since been in touch with authorities over the release of the students.

Witnesses said the armed men came to the Government Science Secondary School in Kankara town at about 21:30 on Friday and that many students jumped the school fence and fled when they heard gunshots.

Many were tracked by the gunmen who tricked them into believing that they were security personnel, students who escaped said. Once the students were rounded up they were marched into the forest by the armed men.