Gunmen have killed three people at an agricultural college in Nigeria's northwest Zamfara state and kidnapped 20 people, including 15 students, school officials and police said Monday, in the latest in a wave of mass abductions, AFP reported.

Armed kidnappings for ransom now make almost daily headlines in Africa's most populous country, seven years after Boko Haram extremists shocked the world by taking 276 girls from Chibok.

Schools targeted by criminal gangs in the north and centre of the West African country are usually in remote areas where students stay in dormitories with only watchmen for security.

On Sunday at the College of Agriculture and Animal Science in Bakura, "unknown gunmen attacked the school around 10 pm (2100 GMT)," registrar Aminu Khalid Maradun told AFP.

They killed a policeman and two security officers, "and abducted 20, including 15 students," he said.

The five other abductees are staff and family members, he added.

The UN estimates that at least 950 students have been  abducted across Nigeria since December.

Most have been released after negotiations with local officials, although some are still being held.

The latest mass abduction was in July, when gunmen kidnapped 121 students from the Bethel Baptist High School in Kaduna state.

Heavily armed criminal gangs known locally as bandits have terrorized communities in the region for years. They usually attack villages to loot, kill and steal cattle.