Human Rights Watch says satellite imagery supports witness accounts that Nigeria's air force struck a refugee camp with "multiple air-dropped munitions," even though tents should have been easily visible.

The New York-based group on Thursday again called for a thorough investigation of Tuesday's bombing of the camp, which one Nigerian official said killed more than 100 refugees and aid workers. The Red Cross says at least 70 people were killed.

Nigeria's military has called the bombing accidental, saying it was trying to target Boko Haram Islamic extremists. The camp's thousands of inhabitants had been displaced by the insurgency, which has killed more than 20,000 people since 2009.

Nigeria's air force says is it investigating.

It is believed to be the first time Nigeria's military has acknowledged making such a mistake.