Five of the 19 soldiers who went missing after an ambush by Boko Haram insurgents Nigeria's volatile northeast have been found, the army has said.

The military launched a search for the troops after they went missing Thursday during a military operation in the town of Alagarno in Borno state, the epicentre of Boko Haram's seven-year Islamist insurgency.

Army spokesman Sani Usman said in a statement late Saturday that five of the soldiers had been found, including the unit's commanding officer.

"Although they are in a stable condition, have been moved to our medical facilities for medical care," he said, adding that the other 14 are still unaccounted for.

Boko Haram's bid to form a hardline Islamic state has left at least 20,000 people dead in Nigeria and displaced more than 2.6 million people, with the violence spilling over into neighbouring Niger, Cameroon and Chad.

A counter-offensive spearheaded by Nigeria since January last year has pushed the militants into remote border areas around Lake Chad.