Suspected Boko Haram militants on Tuesday hurled explosives in Nigeria's troubled northeastern city of Maiduguri, killing five police officers, while a separate blast killed three.

The attacks were the latest to hit the Borno state capital, which is the epicenter of Boko Haram's brutal insurgency which has killed thousands since 2009, including more than 700 this year.

Scores of rioters went on the rampage on Monday outside an abandoned building in Nigeria dubbed the "House of Horror" after decomposing human remains and skeletons were found there.

At least 20 people were injured and 12 vehicles damaged when about 50 people brandishing sticks and machetes were prevented from accessing the property in the southwestern city of Ibadan, an AFP correspondent said.

Gunmen have killed 13 people in an attack on a farming community in central Nigeria in the latest deadly violence to rip through the region, police said on Monday.

A further 10 people were injured in the shooting in the village of Gbajimba on Sunday morning, which Benue state police spokesman Daniel Ezeala blamed on Hausa-speaking Fulani herdsmen.

Boko Haram has claimed responsibility for an attack on a key military barracks in Nigeria, in a new video obtained on Monday by Agence France Presse that warns of further bloodshed, including against civilians.

"We carried out the attacks in Maiduguri (on March 14)," said a man dressed in white, wearing black headgear and carrying an assault rifle, claiming to be the group's leader, Abubakar Shekau.

Nigerian police said Sunday they have opened a murder probe after the discovery of rotting bodies and skeletons in an abandoned building the media has branded a "House of Horror."

Police also rescued several severely malnourished people founded wandering in the bush near the building in the city of Ibadan, while media reports said 15 or more had been found shackled in leg-chains inside.

At least 17 people have been killed by an explosion in a village market in northeast Nigeria in an attack blamed on Boko Haram Islamists, a local police chief said on Sunday.

The deadly blast struck a busy marketplace late on Thursday in the remote village of Nguro-Soye, Borno state, injuring many more, said police chief Lawan Tanko.

Public secondary schools in Nigeria's northeast Borno state have been closed indefinitely following deadly attacks blamed on Boko Haram Islamists, teachers and parents said Saturday.

The closure reportedly affects 85 secondary schools, catering to some 120,000 students across the troubled state, a stronghold of the militant sect waging a five year insurgency in Nigeria.

Turkey's national carrier denied on Wednesday allegations of arms shipments to unknown groups in Nigeria, which has been ravaged by violence between the army and Islamist militants.

"Turkish Airlines, in accordance with company policy, does not ship arms to countries ... where there is a power vacuum or conflict," the airline said in a statement sent to Agence France Presse.

Nigeria on Tuesday unveiled a new "soft power" plan to defeat Boko Haram, hoping to win hearts and minds in the restive northeast, as the military said it was tightening the net on the ground.

National security adviser Sambo Dasuki said the government was to introduce schemes to encourage local communities to shun extremism as well as "de-radicalize" suspected militants awaiting trial.

Nigeria's President Goodluck Jonathan on Monday opened a national conference, calling on delegates to tackle the country's burning issues head-on to build a stronger, more united nation.

Nearly 500 delegates from the hundreds of ethnic, religious and linguistic groups that make up Africa's most populous nation have converged on Abuja for the conference that could last three months.