Nigerian Islamist militant group Boko Haram is reported to have struck again in Borno State, killing hundreds of people in fresh attacks, as the US joins the ongoing hunt for hundreds of abducted school girls.

On Monday night in the town of Gamboru Ngala near Nigeria's border with Cameroon, residents said that heavily armed Boko Haram militants gunned down civilians and set fire to buildings including a police station.

A local senator said hundreds were dead. However, a Nigerian military spokesman said the incident was unconfirmed.

"About 300 persons were confirmed dead after the incident, with several others injured. Almost all the houses in the communities were destroyed by the hoodlums who threw IEDs [improvised explosive devices] at the buildings," Senator Ahmed Zannah, who is from the area, told BBC Hausa.

The attack in Gamboru Ngala happened after security personnel deployed in the town were moved to another area as part of the ongoing operations to find the missing girls, according to local media reports.

The gunmen in armoured vehicles stormed the town, randomly opening fire at the public in a local market.

"They burnt vehicles, cars and 17 trailers loaded with cows and grains in the market," an eyewitness told Reuters, which confirmed 13 deaths.

The militants killed unarmed civilians in the deadly attack that lasted for about 12 hours, Zannah said.

"The attackers stormed the communities in the night when residents were still sleeping, setting ablaze houses and shooting residents who tried to escape from the fire," he said.

"My brother who was at the scene of the attack told me that the actual number of the dead cannot be ascertained but at least they are up to 300. In fact, as he spoke he wept following the high number of the dead bodies which littered the market."

Meanwhile, US President Barack Obama has sent a team of American experts from various law enforcement agencies to help rescue nearly 300 girls kidnapped by the Islamists three weeks ago.

Describing the offensive as "heartbreaking" and "outrageous," Obama said: "We have already sent in a team to Nigeria... a combination of military, law enforcement, and other agencies who are going in, trying to identify where in fact these girls might be."

On Monday, the leader of the militant group threatened to sell the girls, aged between 16 and 18, as slaves.

Nigeria's capital of Abuja is for the first time hosting the World Economic Forum/Africa, which will feature an appearance by China's premier, Li Keqiang, as well as African leaders and the chiefs of major global companies.

The event which is normally held in South Africa has been moved to celebrate the fact the Nigeria's economy is emerging as the most dynamic in the continent.

Increased activity by Boko Haram is being seen as a deliberately defiant and attention grabbing move to catch the glare of the world's media.