A group of gunmen, suspected to be from the Islamist group Boko Haram, opened fire at worshippers Sunday morning when they were offering prayers in Konduga town, about 35 km from Borno capital Maiduguri, according to officials in Borno.
News of the killing emerged Monday.
Media reports said that 26 people were undergoing treatment for injuries sustained during the attack.
Nigeria's President Goodluck Jonathan has declared a state of emergency in three north-eastern states as his troops have been carrying out an offensive against the Islamist militants in the region since May.
Boko Haram wants to overthrow the Nigerian government and create an Islamic state in the north.