Nigeria on Thursday gave details of a formal probe into a botched air strike that killed at least 70 people, as aid workers feared the bloodshed could affect vital humanitarian programmes.

More than 100 people, many of them children, were injured in the bombing at a camp for people displaced by the Boko Haram insurgency in Rann, in the country's northeast, on Tuesday.

Six local Red Cross workers who were distributing food to between 20,000 and 40,000 people living in makeshift shelters at the camp were among the dead.

The Nigerian Air Force said a board of inquiry comprising six senior officers would investigate the bombing, and had initially been presented with a list of 20 witnesses.

"Among its terms of reference, the board is to determine the immediate and remote causes as well as the circumstances that led to the incident," it said in a statement.

The board will submit its report no later than February 2, it added.

Military commanders have already called the bombing a mistake, blaming it on "the fog of war".

They said the intended target was jihadists reportedly spotted in the Kala-Balge area, of which Rann is part.

The Nation newspaper, sympathetic to President Muhammadu Buhari, attributed the bombing to a "failure of intelligence" caused by information provided by a "foreign country", without elaborating.

Boko Haram, which wants to establish a hardline Islamic state in northeast Nigeria, has laid waste to the area since taking up arms against the government in 2009.

At least 20,000 have been killed and more than 2.6 million made homeless.

On December 24, Nigeria said it had flushed Boko Haram fighters from their stronghold in the Sambisa Forest of Borno state and that the group was in disarray. But attacks on troops and civilians continue.

- Two bombs -

Alfred Davies, a field coordinator for Doctors Without Borders (MSF), and one of the injured victims who spoke to AFP, said the air force dropped at least two bombs.

In an account made public by the medical charity, Davies said the first landed just metres (yards) away from the Red Cross office.

"The plane circled back and it dropped a second bomb five minutes later," he said, adding that they were "dropped on houses".

He added: "There are no words to describe the chaos. Some people had broken bones and torn flesh; their intestines hanging down to the floor. I saw the bodies of children that had been cut in two."

MSF arrived in Rann last weekend to vaccinate children and screen for malnutrition, which has gripped the region and left hundreds of thousands of people in dire need of help.

The town, near the shores of Lake Chad and the border with Cameroon, was previously inaccessible because of insecurity, and people were dying of hunger, Davies said.

"The army that was meant to protect them bombed them instead," he said.

The International Committee of the Red Cross (ICRC) said 61 injured people were airlifted to its specialist trauma unit in Maiduguri, the capital of Borno state.

Of those, 28 were children, seven were women and 26 were men, it added.

- 'Huge setback' -

International aid agencies have condemned the bombing of civilians, who are facing extreme food shortages because of the conflict, as well as having lost their livelihoods and families.

One aid worker, who asked not to be identified, described the incident as "horrifying" and "a huge setback to humanitarian work in the northeast".

"It has instilled fear and uncertainty in the humanitarian organisations," the aid worker told AFP.

"The bombing has made the security situation in northern Borno murkier, and international aid agencies who carry out most of the humanitarian interventions more cautious in venturing into remote areas where their interventions are needed most."

That could lead aid agencies to restrict their work to more secure locations, which would leave "a large number" of the displaced without any assistance.

"Aid agencies work in a complementary way in which each focuses on specific humanitarian roles for effective humanitarian services to the displaced," the aid worker said.

"When one or more aid agencies pulls out of a location, humanitarian work suffers because some components will be missing."