Sx years ago, Nigerian forces captured the leader of the extremist group Boko Haram and killed him in custody — a move that was to have far-reaching consequences.

Nigeria had exchanged a charismatic preacher named Mohammed Yusuf for the more radical and violent leader, Abubakar Shekau, who steered the movement down a murderous path. His fighters cut the throats of schoolboys, abducted schoolgirls, seized a slice of northeastern Nigeria and finally threatened to spill war across borders to other West African nations.

In his latest video, released Tuesday, Shekau says God commanded that his fighters massacre hundreds of people in the town of Baga in northeastern Nigeria this month in an attack described by human rights groups as the worst so far by the militants.

In a few short years, Shekau has ruthlessly transformed Boko Haram from a militia that mainly attacked army posts and police stations to one that has launched suicide bombings across the country, targeting civilians in markets, bus stations and clubs. Its fighters have swept into dozens of northeastern villages this past year, massacring civilians and burning houses and shops.

He gained global notoriety last year with his menacing grin, as he boasted about abducting 276 schoolgirls whom he vowed to sell as slaves. And along the way his group gained significant resources and arms, often able to outgun Nigeria’s military.

Claiming responsibility for this month’s massacre, Shekau said in the video on YouTube that the devastation in Baga was nothing compared to future attacks he was planning.

“We are the ones who fought the people of Baga, and we have killed them with such a killing as he [God] commanded us in his book,” said Shekau, as Nigeria and its neighbors struggled to deploy a regional force to confront the violent Islamist group.

“This is just the beginning of the killings,” he said. “What you've just witnessed is a tip of the iceberg. More deaths are coming.”

Satellite images released last week show thousands of houses in Baga and neighboring Doron Baga burned down in the attack. An accurate count of how many were killed has not been reached, but estimates range from several hundred to 2,000 people.

More than 7,500 people fled the assault that began Jan. 3, according to the United Nations. Survivors described Boko Haram gunmen shooting people on sight, or dragging them from their homes and killing them.

Shekau, once a headache for Nigerian President Goodluck Jonathan, now has the presidents of several neighboring countries alarmed — and angry that Nigeria failed to curb the terrorist group earlier. Boko Haram has attacked villages in Cameroon, recruited fighters from several countries and raised the threat of a devastating regional conflict.

Shekau’s birthplace is variously reported as a remote Nigerian village in Yobe state or the neighboring nation of Niger. His parents were peasants. He studied under an Islamic cleric and later attended the Borno State College of Legal and Islamic Studies, where he took Islamic studies and Arabic.

As a student, Shekau was radicalized, became interested in Boko Haram and “began to be abrasive and radical, shunning other students and keeping company of his fellow sect members,” a fellow student, Kayam Bulama, said in an interview last year.

Under his leadership, Boko Haram has wreaked mayhem and chaos in northeastern Nigeria, destroying the economy and agriculture as farmers flee their land, choking off trade between Niger, Nigeria and Cameroon and attacking secular schools. The group, which opposes democracy, is poised to undermine Nigerian elections due next month, with dozens of towns and villages under its control.

The group’s ultimate goal is to impose an Islamic state in Nigeria based on a fundamentalist interpretation of Islam.

In this week’s video, Shekau taunted West African leaders who are working to deploy the regional force, and mocked Niger's president, Mahamadou Issoufou, who took part in a recent rally of world leaders in Paris showing solidarity after the terrorist attack on the satirical magazine Charlie Hebdo.

Baga was to be the headquarters of the 2,800-strong regional force, which was supposed to be launched in November after an agreement involving Nigeria, Niger, Chad and Cameroon. But before the Jan. 3 attack on Baga, Niger’s soldiers had departed and the Chadian forces had not yet been deployed, leaving Nigerian forces to confront the militants alone and further undermining trust among the neighbors.

Chad sent a force to Cameroon last weekend to help prevent the Boko Haram insurgency from spreading across the region, and the African Union is due to discuss a proposed regional force next week.

Ghanian President John Dramani Mahama, chairman of the Economic Community of West African States, told Reuters news service that the African Union might take over the planned multinational force.

“I do believe that dealing with it collectively is a much better way to go about it, because the threat of Boko Haram goes beyond Nigeria. As you can see currently there are attacks taking place in Cameroon, there have been incidents in Niger and so it is not a Nigerian problem only,” Mahama told local journalists during a visit Wednesday to Germany.

“Terrorism is like a cancer and if we don't deal with it, it will keep going. It threatens everybody in the sub-region. When it comes to terrorism nobody is too far or too near,” he told Reuters last week.

“The increase in strength of Boko Haram reflects our slowness and our inability to put up a robust response,” Niger’s foreign minister, Mohamed Bazoum, told journalists last week.

When Boko Haram seized Baga, it took over the military base, along with weapons and ammunition. Shekau flaunted an arsenal of sophisticated weaponry in the YouTube video and said he was ready for any attack.

“Kings of Africa, you are late,” he said. “I challenge you to attack me even now. I'm ready.”