The death toll from a botched air strike on Boko Haram fighters in northeast Nigeria rose to 70 on Wednesday, as aid agencies indicated more people could die without urgent treatment.

Nigeria called Tuesday's bombing of a camp for displaced people in Rann a mistake and blamed the "fog of war", sparking strong condemnation from aid agencies working in the crisis-hit region.

The International Committee of the Red Cross (ICRC), which lost six members of its Nigerian affiliate, said: "It is estimated that 70 people have been killed and more than 100 wounded."

The air strike happened as aid workers distributed food at the military-run camp housing tens of thousands of people.

ICRC surgeon Laurent Singa, part of a team dispatched to Rann shortly after the bombing, described the conditions for post-operative care as "not adequate".

"All the patients must be evacuated to (Borno state capital) Maiduguri as soon as possible," he added in a statement.

Nine patients were said to be in a critical condition and were evacuated to Maiduguri on Tuesday. Forty-six of the 90 that remain were said to be severely injured.

They needed to be transferred "as a matter of urgency", the ICRC said, adding that patients were being treated in the open air in a "precarious environment".

An ICRC spokeswoman in Abuja told AFP: "We are hoping that by nightfall all patients in need of hospital care will be transferred."

- 'Catastrophic event' -

Aid agencies assisting the hundreds of thousands of people in northeast Nigeria in dire need of food, shelter, clean water and healthcare expressed shock and dismay at the bombing.

The UN High Commissioner for Refugees, Filippo Grandi, described it as "a truly catastrophic event", calling for a full investigation to prevent any repeat.

Jean-Clement Cabrol, director of operations for the medical charity MSF, which earlier gave a death toll of 52, said the attack was "shocking and unacceptable".

The secretary-general of the Norwegian Refugee Council, Jan Egeland, said: "It cannot become the new normal that 'accidental' attacks on camps sheltering the innocent are allowed to happen again and again in conflict zones."

Human Rights Watch's senior Nigeria researcher Mausi Segun said the government in Abuja should compensate victims and their families.

"Even if there is no evidence of a wilful attack on the camp, which would be a war crime, the camp was bombed indiscriminately, violating international humanitarian law.

"Victims should not be denied redress merely because the government decided the bombing was accidental."

Accidental bombings have occurred before in the conflict and senior military commanders called the latest "a mistake", saying humanitarian workers were not targeted directly.

Major General Lucky Irabor, who heads the counter-insurgency operation, said the air force jet had been told to target insurgents in the flashpoint Kala-Balge area but hit Rann instead.

A delegation headed by President Muhammadu Buhari's chief of staff Abba Kyari and including armed forces chiefs visited Maiduguri on Wednesday and was also scheduled to visit Rann.

Kyari called the bombing "very unfortunate" and noted "incidents of friendly fire are as old as history of mankind" but said that was no excuse and he was happy there was no cover up.

- Questions asked -

Local and international aid agencies have until recently been unable to get to Rann because of bad roads and insecurity in the remote region near the border with Cameroon around Lake Chad.

The military announced last month it has ousted Boko Haram from its camps in Sambisa Forest, in southern Borno, sending fighters north.

Nigeria's military has announced an investigation into the incident. The Daily Trust newspaper reported that clearly marked ICRC tents were bombed, without quoting sources.

MSF said none of its staff was injured or killed but that three employees of a Cameroonian firm it hired to provide water and sanitation services lost their lives.

Ties have been strained between humanitarian agencies and the Nigerian authorities, which have accused some aid organisations of exaggerating the food crisis triggered by the insurgency.

In December, Save the Children said 4.7 million people in the northeast needed food assistance and some 400,000 children were at imminent risk of starvation.

The presidency called some of the claims "hyperbolic" while the Borno state governor recently accused some aid agencies of profiting from the crisis.