Gunmen believed to be part of Islamist terror sect Boko Haram stormed a federal prison in Koton-Karifi in central Nigeria, killing one guard and freeing 18 inmates.

Prisoners escaped while guards fought against the invaders. Nigeria Prisons Service spokesman Odeyemi said 118 convicts could not be accounted for.

The storming of the prison followed reports of a "deep throat" inside Boko Haram, who alleged that senior figures in the country's establishment were bankrolling the group.

It is still uncertain if the prison held any members of the sect.

The reason for the attack was unknown, Odeyemi said. "It might be that some of the armed robbers are trying to free the armed robbers there awaiting trial."

An investigation into the jailbreak has been launched.

A witness said 20 fighters armed with bombs and guns opened fire on a security man at the entrance of the prison and killed him on the spot. "The main gate of the prison was bombed by the Boko Haram members," said witness Mariamu Bello.

The latest incident follows revelations that Kabiru Sokoto, a member of the sect, has identified its financial backers.

"The man is co-operating well with us," an unnamed source inside the security forces told the Lagos-based Daily Champion.

Sokoto is a suspect in the deadly Christmas bombing of a church near the capital Abuja, which killed 43 people and left dozens injured.

Boko Haram had previously attacked a number of other prisons to free members and also managed to help Sokoto escape police custody, although he has since been rearrested.

He allegedly admitted that security forces are getting closer to the inner circle of the Islamist radical sect.