Gunmen known locally as "bandits" killed at least 140 people in multiple raids this week in northwest Nigeria, four residents told AFP Saturday, but there was no official confirmation.

Northwest and central Nigeria have been plagued by criminal gangs for years but violence has become more widespread now. On Wednesday, the Nigerian government officially labelled bandits as terrorists.

"We buried a total of 143 people killed by the bandits in the attacks," said one of the four sources, Balarabe Alhaji, a community leader in one of the affected villages in Zamfara state.

Hundreds of motorcycle-riding gunmen rampaged through ten villages in Anka and Bukkuyum districts on Wednesday through Thursday, shooting residents, looting and burning homes, locals said.

Officials and security agencies have not yet commented on the attacks. 

Babandi Hamidu, a resident of Kurfa Danya village said the militants who rode into the villages were shooting "anyone on sight".

"More than 140 people were buried across the ten villages and search for more bodies in ongoing because many people are unaccounted for," Hamidu said.

Idi Musa, resident of another village, Kurfa Danya, said "the death toll is huge. We are taking of around 150 people killed by the bandits."

Another resident who only gave his first name Babangida gave a similar account.

All four sources said they were present at burials in their respective villages.