Nigerian troops have rescued two teachers and five students after clashing with gunmen who raided a college in a northwestern state and abducted an unknown number of people, the army said on Friday.

Gunmen stormed the Federal Government College in Kebbi State on Thursday, killing a policeman and snatching students and teachers, in the third kidnap attack on a school in less than three weeks in Nigeria.

Troops in pursuit clashed early on Friday with the gunmen who had split into two groups on the run, the army said in a statement.

Security forces "have so far, rescued two teachers and five students after a fierce exchange of fire with the criminals," it said.

It said one female student had been found dead from exhaustion.

Troops also recovered 800 cattle stolen by the same gang.

Police have so far not confirmed the number of missing students and teachers taken from the school.

Heavily armed criminal gangs, known locally as bandits, have long terrorised central and northwestern Nigeria, raiding and looting villages, stealing cattle and kidnapping for ransom.

But since December they have increasingly targeted schools, arriving on motorbikes, abducting students or schoolchildren and herding them into forest hideouts to negotiate ransom payments.

At the end of May, gunmen seized 136 children from an Islamic seminary in central Nigeria's Niger state. They are still being held.

Nearly 900 children and students have already been kidnapped by gunmen for ransom since December, though many have since been freed.