Suspected Islamist militants killed at least 10 people on Sunday in an attack on a Nigerian village less than 5 km. (3 miles) from Chibok, the scene of a mass abduction of more than 200 school girls in April, survivors said.

In a separate assault on Friday evening, the insurgents killed seven soldiers in the village of Goniri, in Yobe state, a security source and witnesses said.

Violence in Nigeria's northeast has been relentless in the past year, and has gained in intensity since April, the month that more than 200 schoolgirls were snatched by Boko Haram rebels from Chibok. Efforts to free them, which have attracted Western support, have so far not succeeded.

Samuel Chibok, a survivor of the attack on Kautikiri village, in the Chibok community, said that around 20 men in a Toyota pick-up truck and motorcycles rolled into town.

"Initially I thought they were military but when I came out, they were firing at people. I saw people fleeing and they burned our houses," he said, adding that some people had died in the attack, including two of his relatives.

"Smoke was billowing from our town as I left."

A local pro-government vigilante, who declined to be named, said residents had so far recovered 10 bodies from the village.

Boko Haram, which is fighting for an Islamic state in largely Muslim northern Nigeria, has killed thousands since launching an uprising on 2009, and many hundreds in the past three months.

It is by far the biggest security threat to Africa's biggest economy and top oil producer, and has overshadowed government efforts to project an image of Nigeria as prospective economic giant and massive investment prospect.

An explosion on Friday night in a brothel in the northeastern Nigerian city of Bauchi killed 11 people and wounded 28, police said on Saturday, an attack also believed to be the work of Boko Haram.

A military operation in the northeast has so far failed to quell the rebellion and has triggered reprisal attacks that are increasingly targeting civilians, after they formed vigilante groups to try to help the government flush out the militants.

But their tactics - often striking then fleeing over the border into Cameroon - have repeatedly proved devastating. They are well armed and funded by a lucrative kidnapping operation.

In Friday night's attack on a military outpost, suspected Boko Haram fighters arrived in four armoured personnel carriers and 11 hilux trucks mounted with anti-aircraft guns, said a security source and a witness who gave his name only as Hamisu.

"They were all dressed in full military but they did not direct their onslaught on the civilian population," Hamisu said by telephone.

The militants are extending their reach beyond their remote northeastern heartlands. A bomb in an upmarket shopping district of the capital Abuja killed 21 people on Wednesday, the third attack on the capital in three months.

President Goodluck Jonathan said Nigeria had entered one of the darkest phases of its history during a visit to the scene of the Abuja blast on Friday.