The army eventually admitted ordering the strike near the town of Rukubi, in a central region known for ethnic and religious violence, that hit a group of herders on January 24.

But in a response to Human Rights Watch (HRW) dated May 17, the army said it had targeted "alleged terrorists" based on "credible information", without admitting any fault.

"The military's unacceptable delay in owning up to the killing and injuring dozens of civilians only compounds the tragedy of this shocking attack," HRW's Africa researcher Anietie Ewang said in a statement.

"The Nigerian military should provide full accountability for their actions as well as financial compensation and livelihood assistance commensurate with the needs of the victims and their families," she said.

In Washington, State Department spokesman Vedant Patel said, "Reports of civilian casualties should be thoroughly and transparently investigated."

Efforts to prevent civilian casualties "are central to our security cooperation with the Nigerian military", Patel told reporters.

Nasarawa State Governor Abdullahi Sule earlier told Nigerian media that the attack was carried out by a drone.

Army officials declined to comment immediately when contacted by AFP on Tuesday.

Security forces regularly carry out airstrikes in northwest and central Nigeria against Islamist insurgents as well as criminal gangs, at times accidentally hitting civilians.

Human Rights Watch said it based its investigation on interviews in March with two survivors and seven family members of the victims.

It said it also examined photographs of bodies and visited a mass grave of 31 bodies.

Alhaji Hassan Bello, who said he lost nine loved ones during the attack including his three sons and six brothers, told the rights group that he fainted when he saw the bodies and was hospitalised for three days.

"When I close my eyes, I hear their voice and then I see their bloodied bodies," he was quoted as saying.

HRW said that witnesses described officers seizing more than 1,000 cows two weeks before the incident after they strayed into Benue State.

Herders said they paid fines but believe they were deliberately targeted as a reprisal, with authorities in Benue hostile to herders from the Fulani community.