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Just over a year after Nigeria proudly declared Boko Haram was all but defeated, its fighter jets mistakenly bombed a camp full of people driven from their homes by the militants.

The airstrike illustrates the insurgents — the world's deadliest terrorist group during 2015 — are still far from subdued.

The site of a bombing attack at a camp for internally displaced persons in Rann, Nigeria, on Tuesday. MSF via Reuters

Estimates for the number killed in Tuesday's botched operation outside the northeastern town of Rann ranged from 50 to more than 100.

Filippo Grandi, the United Nations high commissioner for refugees, on Wednesday called the incident a "truly catastrophic event" and called for a full investigation.

In a rare public apology, Nigeria's President Muhammadu Buhari offered his condolences to the families, labeling the bloodbath a "regrettable operational mistake."

Buhari first proclaimed in December 2015 that he had "technically defeated" Boko Haram, the violent Sunni Muslim sect that has waged war in northeast Nigeria since 2009.

He has made similar claims in the 12 months since, and anyone listening to these speeches might be moved to ask: If Boko Haram has been defeated, who are Nigeria's jets trying to bomb?

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The Nigerian military has been successful in liberating almost all of the territory once held by Boko Haram, but the country's northeast is still peppered with hit-and-run attacks and suicide bombs.

Last year, a split emerged in the group. One faction is now led by the enigmatic Abubakar Shekau, one of America's most-wanted terrorists with a $7 million reward for information on his head, and the other by Abu Musab al-Barnawi, who has received the backing of ISIS.

The military appears to have had some success against Shekau's faction, but less against al-Barnawi's group, according to J. Peter Pham at the Atlantic Council, a Washington-based think tank.

"More worryingly, this faction is displaying complex tactics and weapons, and that suggests outside influence," said Pham, who has just returned from the region. "My reading is that ISIS is being squeezed in Libya and it is coming south to Nigeria."

The town of Rann is near Nigeria's border with Cameroon. Google Maps

Pham said the army has improved its counterterrorism strategy and enjoyed gains against Shekau's faction along a southeastern front near the border with Cameroon. "They don't get enough credit for that," he said.

But it's had less success in the north near the Niger border, where Pham said the newer, ISIS-backed faction is operating.

He said the bombing also showed a need for Nigeria to up its intelligence game. "Granted, accidents happen in warfare but this was a huge error," he said.

A map posted by Human Rights Watch showed the camp for displaced people clearly visible from the air.

Stunning miss: Camp bombed by #Nigeria easily seen from air--packed with tents & next to military base, notes @hrw sat image pro Josh Lyons. pic.twitter.com/qF3Fjx6ZAK — Letta Tayler (@lettatayler) January 18, 2017

It's not just the ongoing fighting that means Nigeria remains in a state of crisis.

The legacy of the group's pitiless seven-year insurgency is also being felt more than ever, with much of the region's farming and trade having been obliterated.

Boko Haram came into international view in 2014 after kidnapping more than 270 schoolgirls from the town of Chibok, an assault that provoked the #BringBackOurGirls campaign supported by Michelle Obama and many Western celebrities.

Our prayers are with the missing Nigerian girls and their families. It's time to #BringBackOurGirls. -mo pic.twitter.com/glDKDotJRt — The First Lady (@FLOTUS) May 7, 2014

But its insurgency has been raging since 2009, racking up a death toll of around 20,000 and making Boko Haram the world's deadliest terror group in 2015.

As of last week, the U.N. now estimated there were 2.2 million people displaced from their homes, most of them children, and some 7.1 million in desperate need of humanitarian assistance, more than double a year ago.

The situation hasn't been helped by collapsing oil prices that contributed to Nigeria plunging into recession last year.

Unless humanitarian assistance is provided immediately, the country will see "a famine unlike any we have ever seen anywhere," Toby Lanzer, the U.N.'s assistant secretary-general in the region, said last year.

He added: "The situation of many affected communities has deteriorated beyond alarming levels. If we do not act fast, and do more especially in areas that were previously inaccessible, thousands of people will die."