DAKAR, Senegal » Thousands of buildings were burned, damaged or destroyed in northern Nigerian towns in recent days when Boko Haram militants stormed through, using scorched-earth tactics against civilians, according to a new analysis of satellite images by human rights groups.

In a succession of attacks, fighters from Boko Haram, an Islamist insurgent group that has gripped northern Nigeria and battled the government for years, have swept through a cluster of villages along the shores of Lake Chad in a "systematic campaign of arson directed against the civilian population in the area," according to Human Rights Watch.

About 57 percent of one town, Doro Gowon, the location of a now-destroyed military base, appears to have been leveled, probably amounting to several thousand residential and commercial structures, Human Rights Watch said.

Amnesty International, which has also analyzed the satellite images, said Thursday that about 3,100 buildings in the town had been damaged or destroyed, demonstrating a "deliberate attack on civilians whose homes, clinics and schools are now burnt-out ruins."

Massacres of civilians by Boko Haram shook northern Nigeria throughout 2014. About 300 people were killed in Gamboru Ngala in May, perhaps 500 died in a group of villages in Borno state in early June, an additional 120 died in Kano in November and more than 150 died in Damaturu in December, along with numerous others.

In the first six months of 2014 alone, Boko Haram killed more than 2,000 civilians in northern and central Nigeria, Human Rights Watch estimated last year.

Some of these massacres have attracted little attention. But when militants raided the fishing village of Baga in early January, Amnesty International asserted early on that "as many as 2,000" were killed. That figure has been impossible to verify, however, because the area quickly fell under the control of Boko Haram, causing thousands to flee.

Witnesses agreed that many had been killed in Baga when Boko Haram came riding in, guns blazing and shooting indiscriminately at civilians, as has been their custom, at least since 2011. They said the killing began Jan. 3, after the militants overpowered Nigerian soldiers at a military outpost there. Then the fighters turned on the residents.

But estimates of the death toll are murky.

"It’s not possible to get any credible figures out of there," said Mausi Segun, Human Rights Watch’s Nigeria researcher. "It’s impossible to even guess."

Amnesty International appeared to be backing away from its initial 2,000 figure, saying Thursday that witnesses and local officials suggested that "Boko Haram militants shot hundreds of civilians."

The area has suffered deadly attacks before — from both sides in the conflict.

In 2013, Baga was attacked by the Nigerian army, with heavy civilian casualties. Enraged soldiers felt the populace was too close to Boko Haram at that time. Satellite images then showed that some 2,275 buildings had been destroyed.

Survivors and witnesses said about 200 people had been killed in the army massacre, leading to broad, international criticism of the Nigerian government’s counterterrorism campaign in the region.

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