A group of Boko Haram militants is reported to have killed 37 people and burned down six northeastern Nigerian villages in an attack near the insurgents' stronghold of Sambisa Forest.

A fleeing survivor said many victims of Wednesday night's attacks were farmers who had recently returned home after soldiers earlier this year forced the extremists out of the area where they had declared an Islamic caliphate.

Now they are refugees again, with many survivors suffering gunshot wounds and burns, said Ahmed Ajimi, a fighter in the anti-Boko Haram Nigerian Vigilante Group.

"It was really horrifying," said Ajimi, who described the insurgents shooting from the back of pickup trucks and hurling firebombs that quickly set ablaze the thatched roofs of scores of huts.

He said he spent the night in the bush and returned on Thursday to help bury 37 corpses.

Boko Haram last week denied Nigerian military reports that militant fighters were hemmed into the Sambisa Forest and claimed its fighters still held towns hundreds of miles away. There was no way to verify the competing claims.

The Islamic uprising is estimated to have killed some 10,000 people just last year when it began attacking neighboring countries as well as Nigeria. A multinational force this year drove Boko Haram fighters out of towns and villages they had held for months.

Thousands of Nigerians have been killed by Boko Haram and at least 1.5 million people have been displaced by the conflict.

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Human rights groups have also documented extensive abuses by the Nigerian military against civilians, including young children. Amnesty International said earlier this month that more than 7,000 young men and boys have died in military detention since March 2011, while at least 1,200 people have been unlawfully killed since February 2012.

A summit of Nigeria and its neighbors on Thursday in Abuja, the Nigerian capital, pledged to increase the security force to 8,500 troops by the end of July with Nigeria pledging $30 million for the installation and equipping of its headquarters in N'Djamena, Chad's capital.

The leaders agreed to Nigerian President Muhammadu Buhari's suggestion that a Nigerian general would command the force with his deputy from Cameroon and the chief of staff from Chad.