An army spokesman said the death toll stated by the sources was not true.

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.

"It is not true we lost that number," said army spokesman Sani Usman in a text message.

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.