At least 25 people died when suicide bombers attacked a mosque in the country's north-east in the early hours of Wednesday.
The attack - in the village of Umurari, near Maiduguri, the capital of Borno state - came during morning prayers, a witness at the scene told dpa.
"The explosions occurred during early morning prayers, killing about 25 people and injuring about 42 others," Isah Salihu said.
Salihu said security at the mosques was scaled down only two days ago following a growing feeling that the insurgent group, Boko Haram, no longer posed a risk to worshippers.
According to Nigeria's Daily Trust newspaper, the attack was carried out by unidentified suicide bombers who entered the mosque during prayers without being detected.
A female suicide bomber disguised as a man detonated one bomb inside the mosque while another bomber waited outside, the newspaper said.
When rescuers rushed to the scene, the other bomber detonated her explosive.
The Nigerian-based group, which seeks to implement its own radical version of Islam, has killed thousands in north-eastern Nigeria, Cameroon, Niger and Chad since 2009.