The Nigerian army, aided by troops from neighbouring countries, has, since early last year recaptured most of that territory
Thirteen Nigerian soldiers were wounded in clashes with the Islamist militant group Boko Haram and others are missing, the army said on Tuesday.
Army spokesman Sani Usman said the clashes took place at Gashigar, a town in the northeast's Borno state, near the border with Niger.
"The troops did their best to defend the location in vain. In the process, 13 soldiers sustained injuries while some are still missing in action," said Usman. He did not say how many were missing.
Islamic State (IS), to which the Nigerian jihadists have pledged allegiance, said on Twitter that a total of 20 soldiers from Nigeria and neighbouring Niger had been killed and many others were wounded. Usman denied this.
Boko Haram has waged a seven-year insurgency in a bid to create a caliphate in the northeast where a strict interpretation of Islamic laws would be observed. During that time 15,000 people have been killed and two million displaced.
It controlled a swathe of land in northeast Nigeria about the size of Belgium at the end of 2014.
The Nigerian army, aided by troops from neighbouring countries, has, since early last year recaptured most of that territory. But the group still stages suicide bombings in the northeast, as well as in neighbouring Niger and Cameroon.
Divisions within Boko Haram were exposed in August when a new leader of the group was announced by IS only for Abubakar Shekau, the Nigerian jihadist's traditional figurehead, to reject the move in statements circulated on social media.
Two factions of Boko Haram -- one loyal to IS and the other loyal to Shekau -- are based in northeast Nigeria.