Nigeria's military said on Wednesday that there has been no evidence of civilian casualties from an airstrike on militants in the northwest Zamfara state this month, calling reports of large death tolls unverified and misleading.

Amnesty International said this week that at least 100 civilians were killed in the May 10 airstrike on a crowded market in Tumfa village, urging authorities to open an immediate investigation.

Citing witnesses, Amnesty said that many of those killed were women and children. Local media also reported a similar death toll.

"No credible, substantiated evidence of civilian casualties has been established through any official assessment or independent verification," Defence Headquarters spokesperson Major-General Michael Onoja said in a statement.

He said the strike was conducted under international humanitarian law and targeted a "confirmed high-level gathering" of militant leaders in the village which was based on multi-sourced intelligence.

Onoja added that the nature of the strike meant that immediate casualty verification was difficult, but a post-strike assessment showed that "several terrorists were neutralised."

Nigeria's military has been battling bandits in the northwest but often describes them as terrorists. It is also battling a 17-year Islamist insurgency in the northeast.