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.

At least six people have died in Nigeria's commercial capital of Lagos after the collapse of a high-rise building that was under construction, the state emergency services chief said on Tuesday.

Gunmen have killed three security agents in northwest Nigeria's Zamfara state, a spokesman said Saturday, in the latest violence in the volatile region.

Nigerian forces have killed 78 gunmen, known locally as bandits, during military operations including air strikes in northwestern Zamfara state, the air force said.

Thirteen policemen were killed in an ambush by cattle thieves in northwest Nigeria's Zamfara state, in the latest spate of violence in the region, police said late on Sunday.

About 125 students were still missing on Wednesday but 28 others had been re-united with their families after an attack by armed men on a boarding school in Nigeria's Kaduna state, the head of the Kaduna Baptist conference said.

Police in northwest Nigeria's Kebbi state told AFP Sunday that the death toll from an attack by a gang of cattle thieves on seven villages Thursday had risen to 88.

Nigerian authorities said on Tuesday that about 200 children had been seized in the latest school kidnapping in its violent north but they ruled out securing their freedom by paying a ransom.

A Nigerian bandit chief who led the kidnapping of hundreds of schoolchildren has been killed in armed clashes with a rival gang, days after reneging on an amnesty deal with authorities, sources told AFP.

Militants overran an army base in northeastern Nigeria, killing more than 30 soldiers before pulling back in the face of air strikes, soldiers and a resident said.

All 279 girls kidnapped from their boarding school in northern Nigeria have been released and are on government premises, the local governor told AFP Tuesday.

Dozens of the 344 kidnapped boys freed from their captors in northwest Nigeria were seen arriving at local government buildings in the city of Katsina on Friday, a Reuters witness said.