Heavily-armed fighters believed to be from the Islamic State West Africa Province (ISWAP) opened fire on a military convoy, 35 kilometres (22 miles) west of the town of Monguno where the soldiers were based.

"The terrorists killed eight troops in the ambush which happened around midday," said a military officer, speaking on condition of anonymity.

"Five more soldiers are still unaccounted for," he told AFP.

The patrol was heading to the regional capital Maiduguri when they came under "heavy fire from the terrorists who used RPGs (rocket-propelled grenades) and heavy guns," said a second officer, also speaking on condition of anonymity.

The militants destroyed two army trucks and a pick-up van fitted with a machine gun, before seizing weapons and withdrawing into the bushes, according to the army officials.

Soldiers who arrived after the attack briefly closed the highway while a Nigerian Air Force jet patrolled the skies in search of the attackers, civilian eye witnesses told AFP.

A decade-long militant insurgency in northeast Nigeria has killed more than 27,000 people and forced about two million to flee their homes.

In 2016, ISWAP split from Boko Haram and longtime leader, Abubakar Shekau.

Since July last year, the group has intensified its attacks on Nigerian military targets.

On August 17, ISWAP fighters killed four soldiers in an ambush on a military patrol team in Mogula village close to the border with Cameroon.

On Tuesday, gunmen from the group shot dead 11 construction workers in Wajirko village, 150 kilometres west of Maiduguri, in a rare attack on civilians.

In recent weeks, ISWAP fighters have also been asserting their presence in areas near the border with Cameroon, controlled by Shekau's group.