KANO, Nigeria (AFP) - A female suicide bomber set off her explosives and the other was killed by troops outside the northeastern Nigerian town of Maiduguri, where a twin suicide attack last week killed 25 people, vigilantes said.

The two were spotted by civilian vigilantes helping the military fight the Boko Haram Islamist group at Molai around 1.00 am, vigilante Babakura Kolo told AFP.

"When the vigilantes flashed their torchlights on them and demanded to know their identities, one of the women detonated her explosives while the other one ran and hid in an abandoned building," he said.

He said soldiers rushed to the scene on hearing the explosion where they joined the vigilantes in searching for the other bomber.

She was found holed up in the building and the soldiers "shot and killed her because she was trying to set off her explosives", said Kolo, whose account was supported by Musa Ari, another vigilante.

Army spokesman Sani Usman confirmed the incident.

"The troops intercepted two female suicide bombers at about 1.20 am this morning, almost 100 metres south of Umurari village (on the) outskirts of Maiduguri," he said.

"However, one of the suicide bombers... detonated her explosive device, killing herself instantly, while the second bomber in an attempt to escape was gunned down," he said.

He said security agents later detonated the unexploded explosives strapped on the second bomber. There were no casualties apart from the pair.

Molai, which lies six kilometres from Maiduguri, has been repeatedly attacked by Boko Haram.

On March 16, two female suicide bombers disguised as male worshippers struck at a mosque in the area during morning prayers, killing 25 people and injuring more than 32 others.

Boko Haram has been carrying out suicide bombings often using female bombers and girls as part of its more than six-year armed campaign to establish an Islamic state in northern Nigeria.

The violence has claimed 17,000 lives and displaced 2.6 million from their homes.