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

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.

Another school official confirmed the attack.

"The gunmen forced themselves into the school through the pedestrian gate and took away the hostages," Abdullahi Aminu, in charge of accommodation at the school, told AFP. 

"They killed three security personnel in a shootout."

The official said the school received a phone call early Monday from the kidnappers, saying they were holding 20 hostages.

Police said on Monday afternoon that 15 students were kidnapped in the attack but said only four staff were taken -- and three were later "rescued".

"15 students and 4 staff were abducted by the bandits," local police spokesman Mohammed Shehu said in a statement, adding that police operatives rescued three staff while searching the area. 

The bandits were "heavily confronted" by security officers during the Sunday night attack, Shehu added, but a "police inspector" and two "civilian guards lost their lives."

950 abducted since December 

A recent school graduate, Usman Usman, also confirmed the incident.

"I left the school on Sunday around 5:30 pm... This morning my friend called me and said that some bandits attacked the school in the night. They killed people and kidnapped many," said Usman, 22.

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 terrorised communities in the region for years. They usually attack villages to loot, kill and steal cattle.

Military deployments and peace deals have failed to end the violence, and bandits who hide in camps in the Rugu forest straddling Katsina, Kaduna, Zamfara and Niger states continue to launch attacks.

Since late last year, the gangs have increasingly targeted schools and colleges to squeeze out ransom cash. 

Bandits chase financial gain and have no known ideological leanings, but there has been growing concern among security experts and officials over potential ties with jihadists in the northeast of the country.

Kidnappings are just one of the challenges facing Nigeria's security forces who are battling a grinding Islamist insurgency in the northeast and separatist tensions in parts of the south.