At least 12 Nigerian soldiers were killed and dozens of others are missing after fighting with Islamists in the northeastern state of Borno, three military sources said on Sunday, one of the largest known losses of life by the army in the last month.
The army said it repelled the attack in which it said one soldier was killed and another injured.
The fighting followed an attack on Friday by insurgents in Gudumbali local government area - a part of Borno where Boko Haram breakaway group Islamic State in West Africa (ISWA) is influential. The sources said the poor communication network in the remote area delayed details of the attack being relayed.
The attack, which the sources say was carried out on a military base and a nearby community in the Gudumbali local government area, comes as President Muhammadu Buhari's security record has become a campaign issue two months ahead of an election in which he is seeking a second term.
The three military sources, all of whom are soldiers and did not want to be named, said at least 12 troops were killed and dozens were missing after the fighting.
The army, in an emailed statement, said the insurgents opened fire on troops while aid materials were being distributed in Gudumbali.
"The troops, however, fought gallantly and outmanoeuvred the attackers inflicting heavy casualties on them. Unfortunately, a soldier paid the supreme price during the encounter, while another was wounded in action," said the army in the statement.
The biggest loss of military life in the last few months occurred in late November when around 100 soldiers were killed by ISWA militants who attacked an army base in Metele, Borno state.
One of the sources said 28 militants were killed.
Nigeria's northeast is home to two Islamist insurgencies: Boko Haram and ISWA which broke away in 2016 and is now considered by security experts to be the stronger of the two.
The Nigerian government in June ordered thousands of people who fled the conflict with Boko Haram to return to Gudumbali, one of the most dangerous areas of northeast Nigeria. Officials cut off food and other aid to those who refused.