Scores of injured people were airlifted on Wednesday to hospital for treatment after a botched air strike on Boko Haram Islamists in Nigeria killed at least 52 civilians and aid workers.

The medical charity Doctors Without Borders (MSF) said more than 120 people were wounded Tuesday in the bombing of a camp in Rann, in the far north of Borno state, the epicentre of the jihadists' insurgency.

Six Nigerian Red Cross workers were among the dead, while 11 others were injured, according to the International Committee of the Red Cross (ICRC).

The Borno governor Kashim Shettima has ordered public hospitals and doctors in the state capital, Maiduguri, to be on standby to receive casualties.

But there were already reports that some casualty departments were overwhelmed.

Some of the injured were being taken to a unit run by the ICRC at the Borno State Specialist Hospital, which normally treats patients wounded in Boko Haram suicide and bomb attacks.

Shettima said three hospitals had been "all placed on standby in readiness to carry out emergency treatment", and that the most critically injured people were being evacuated first from Rann by helicopter.

- Aid agencies shocked -

Accidental bombings have occurred before in the conflict, which began in 2009 and has left at least 20,000 dead and made more than 2.6 million others homeless.

In March 2014, an air force jet killed five and wounded several others when it mistakenly bombed a village in the Konduga area of Borno.

In January that year, the convoy of a Nigerian senator was fired on, after pilots mistook his six-vehicle convoy under police and military escort for Boko Haram fighters.

Senior military commanders in charge of the fight against Boko Haram called the latest bombing "a mistake" and maintained that humanitarian workers were not targeted directly.

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

He blamed the error on "the fog of war".

The aid workers were distributing food at the military-run camp housing tens of thousands of people. Many are suffering from severe malnutrition and acute food shortages because of the conflict.

Jean-Clement Cabrol, the director of operations for MSF, called the attack "shocking and unacceptable". "The safety of civilians must be respected," he said.

Toby Lanzer, the UN humanitarian coordinator for the Sahel region, told AFP: "Never in my 20 years of work in crisis setting have I seen such an incident.

"For aid agencies and their staff tragedy moves us to engage and respond in support of survivors and the communities on the ground acting as first responders."

Local and international aid agencies have until recently been unable to get to Rann because of bad roads and insecurity in the remote region 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.

- Strained ties -

Nigeria's military has announced an investigation into what happened. 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 disclosed that three employees of a Cameroonian firm it hired to provide water and sanitation services were killed.

One aid worker told AFP colleagues were "stunned" at what happened and suggested civilians were likely to have been caught up in previous bombing raids in the remote region.

"I'm sure it (the bombing in Rann) is an accident but why would they (the Nigerian military) bomb a place that they're guarding?" the aid worker said on condition of anonymity.

Ties have been strained between humanitarian agencies and the Nigerian authorities, who 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.