At least 20 people, mostly students, have been killed by unknown gunmen in Mubi, north-eastern Nigeria.

Officials say most of the dead were students who attended the Federal Polytechnic Mubi.

A college spokesman said 26 students had been killed, while a local resident told the BBC that more than 40 people had been shot dead or stabbed.

The killings come days after a major operation against the Boko Haram militant group in the town.