Boko Haram pledged allegiance to the Islamic State group in an audio message published online on Saturday, after three bombs wreaked havoc in northeast Nigeria, killing 58 and wounding scores.

"We announce our allegiance to the Caliph of the Muslims, Ibrahim ibn Awad ibn Ibrahim al-Husseini al-Qurashi," Boko Haram leader Abubakar Shekau said, referring to IS leader Abu Bakr al-Baghdadi.

Three bombings, including one by a female suicide attacker, killed at least 58 people and wounded 139 others in northeast Nigeria on Saturday, in the latest violence blamed on Boko Haram.

Many children were among the dead in the explosions that hit two crowded markets and a busy bus station in Maiduguri, the region's largest city and capital of the embattled Borno state.

When Boko Haram attacks made life at home unbearable, Rebecca Samuel was confronted with a stark choice that any mother would struggle to make.

Her family had decided to leave the remote town of Chibok, northeast Nigeria, in March 2014 because of the persistent threats from the Islamist militants.

An American woman kidnapped by masked gunmen in central Nigeria last month was released on Friday and handed over to U.S. government officials, police told AFP.

Phyllis Sortor, a missionary with the Free Methodist Church, was seized on February 23 in the village of Emiworo in Kogi state.

The African Union has endorsed the creation of a regional force of up to 10,000 men to join the fight against the Nigerian Islamist group Boko Haram, diplomats said Friday.

The force, the idea for which was adopted at an AU summit in January, will be based in Chad's capital N'Djamena, the pan-African bloc's Peace and Security Council said.

Nigeria's government on Friday said that work had begun to rebuild a school in the northeastern town of Chibok from where Boko Haram gunmen kidnapped more than 200 girls last year.

Finance minister Ngozi Okonjo-Iweala laid the foundation stone at the Government Secondary School on Thursday on behalf of President Goodluck Jonathan, a statement from her office said.

Boko Haram gunmen killed 68 people, including many children, in a massacre in the northeast Nigeria village of Njaba, two witnesses and two vigilantes told Agence France-Presse Thursday.

Heavily armed militants stormed the village in Borno state on Tuesday "from all directions", said Falmata Bisika, 62, who escaped to the state capital Maiduguri where she spoke to reporters.

Police in northeast Nigeria on Wednesday said they were hunting the mob who beat a woman to death and burnt her body after suspecting that she was a suicide bomber.

The woman, identified by her mother as 33-year-old Thabita Haruna, was killed on Sunday in Bauchi after she allegedly refused to undergo a security screening at a city market.

Nigerian troops backed by civilian vigilantes on Tuesday claimed to have killed more than 70 ‎Boko Haram fighters in a foiled attack to capture the strategic town of Konduga near Maiduguri.‎

A would-be suicide bomber was shot and killed before he could ram his explosives-laden car into a military checkpoint in the town during the attack on Monday, witnesses said.

Boko Haram gunmen attacked an island on the Niger side of Lake Chad, with locals reporting heavy casualties among those who fled but no official confirmation of the death toll.

A private radio station in Niger said that two people died in a fire in the remote village on Sunday evening but one survivor spoken to by Agence France-Presse said the death toll was higher.