More than 45 people have been killed by two female suicide bombers at a crowded market in the Boko Haram stronghold of Maiduguri in northeast Nigeria, according to witness accounts and security officials.

Health worker Dogara Shehu said he counted more than "45 people killed, some of them completely decapitated" in an account backed up by a witness, who declined to give his name.

A female suicide bomber exploded in the crowded market in Maiduguri, a stronghold of Islamist group Boko Haram, while standing near a rickshaw, a senior security source told AFP.

A second bomb was detonated 10 minutes later by another female suicide bomber who concealed explosives under her hijab and strapped to her back, pretending to be carrying a baby, witnesses said.

"The woman wrapped the explosives on her back, just like a baby and manoeuvred her way to the scene of the earlier explosion," market merchant Abubakar Bello told AFP, whose account was supported by three other people.

The attack targeted the popular market area and the sound of the blast could be heard around the city centre, with thick plumes of black smoke billowing out of the market.

"While the people were trying to help the injured the second bomb blasted," another witness, Sani Adamu, told Reuters.

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"I saw lots of bodies."

A spokesman for Nigeria's National Emergency Management Agency (NEMA) confirmed that "many people have been killed" but did not have an official death toll.

There was no immediate claim of responsibility but Boko Haram has attacked Maiduguri dozens of times during its five-year uprising aimed at creating a hardline Islamic state in northern Nigeria.

The militant group was founded in Maiduguri more than a decade ago and using female suicide bombers has become a growing tactic of the group.

The city was once the epicentre of the conflict, with near-daily clashes between troops and suspected Islamist cells, but some of the fighting has drifted to more remote parts of the northeast.

Maiduguri is the capital of Borno state, where Boko Haram is believed control more than two dozen towns and villages, which the group says are now part of an Islamic caliphate.

Nigerian president Goodluck Jonathan plans to ask the national assembly to extend a state of emergency in three north-eastern states worst hit by the insurgency when it expires this week.

AFP/Reuters