Fighters from the Islamic State West Africa Province (ISWAP) in several trucks opened fire on troops at a checkpoint outside the town of Mainok in Borno state on Wednesday, they said.

"We lost five soldiers in the surprise attack on the checkpoint," a military officer, who did not want to be named, told AFP.

Troops from the town intercepted the militants, leading to a gunfight in which "some of the terrorists" were killed and two vehicles fitted with machine guns were recovered, he said.

Another military source said the five soldiers manning the checkpoint were "outgunned" by the insurgents.

The jihadists were said to have taken three military vehicles away.

The army said late Wednesday its troops killed nine "Boko Haram terrorists" in an ambush, outside Mainok.

Spokesman Sagir Musa said two soldiers were "slightly wounded" in the encounter, without mentioning any fatalities from troops.

Mainok, which lies along the 120-kilometre (75-mile) highway linking Maiduguri and Damaturu, the capital of neighbouring Yobe state, is an ISWAP stronghold.

The area has seen an upsurge in abductions of civilians, prompting increased military deployments to confront the insurgents.

ISWAP which split from Boko Haram in 2016, has intensified attacks against the military since the middle of 2018.

In January, 22 soldiers were killed in three separate ISWAP attacks in the area, according to military sources.

The decade-long jihadist conflict has killed 36,000 people and displaced around two million from their homes in northeast Nigeria.

The violence has spread to neighbouring Niger, Chad and Cameroon, prompting a regional military coalition to fight the insurgents.