It was just one incident during a particularly bloody weekend in Nigeria's ongoing conflict against bandits -- criminal groups that raid villages and impose taxes in areas with little government control.

Armed gangs also killed 30 travellers in a separate attack in northwestern Zamfara state Sunday, according to a security report prepared for the UN and seen by AFP.

Also Sunday, bandits launched "coordinated attacks" in Katsina state that killed 12, according to another UN security report.

The airstrikes in Niger state took place in Shiroro local government area, home to known hideouts for a Boko Haram jihadist faction, as well as non-ideological bandits.

The military denied reports of civilian casualties in the town of Kusasu but said it would investigate.

"On Sunday, an airstrike by the Nigeria airforce targeting terrorists hideout in Kusasu settlement has mistakenly fell on people and killed about 13," Shiroro local government council chairman Isyaku Bawa told AFP.

"It was not intentional. I commiserate with the family of the victims," he said.

A local resident who lost his daughter and two other family members told AFP they buried 13 bodies.

"Others that were injured have been taken to hospital," he said.

"My own daughter, my elder brother's daughter and my elder sister were all killed."

Another resident, John Ezra, said the villagers were "not close to the hideout of the terrorists but our homes were bombed."

He confirmed the toll at 13.

- Military denies casualties -

In a statement over the weekend, the military said it had "conducted multiple airstrikes on different bandit locations including Katerma, Bokko, Kusasu and Kuduru villages".

"About 70 bandits were hit and killed in Kusasu," said Defence Headquarters spokesman Major General Michael Onoja.

Onoja said that "all civilian communities within the general area" had "voluntarily relocated" ahead of the strikes.

"This fact alone fundamentally undermines the claim that innocent residents were present in the strike zones," he said.

"Nonetheless, relevant military formations have been directed to verify the allegations of civilian casualties, if any."

Hundreds of civilians have been killed in airstrikes during Nigeria's 17-year war against jihadists, as well as its separate campaign against bandit gangs.

On Sunday, a Nigerian militia member told AFP that dozens of fishermen were feared dead after continuous strikes by Chadian forces since Friday on Lake Chad, which straddles the two countries.

In April, the Nigerian military struck a crowded market in Jilli, on the border of northeastern Yobe and Borno states, killing at least 56 people, many of them civilians.

In Sunday's attack in Zamfara state, bandits ambushed travellers in Gusau local government area, killing "civilians, hunters and a community protection guard", the report said.

Security forces, responding to the incident, killed "several bandits".

Bandits had previously ambushed and kidnapped passengers travelling in the area on April 25, it said.

The security report said bandit attacks in the southern part of Zamfara have persisted "at the start of the rainy season" as armed gangs continue to exploit "slower security response times".