On Friday, troops launched a ground offensive on a stronghold of Islamic State-affiliated militants in a wasteland straddling Borno and Yobe states.

"The suicide attack killed 27 soldiers, including the commander, and left several others seriously injured," a military officer said, making this one of the deadliest suicide attacks targeting soldiers in recent years.

"It was dark, which made it difficult for the troops to have a clear view of the surroundings," said another officer about the attack which happened around 2030 GMT.

He said the death toll could rise as some of the wounded were in a "critical condition".

A suicide bomber rammed his explosives-laden vehicle concealed in thick foliage into a convoy of advancing troops targeting the Islamic State West Africa Province (ISWAP) in Timbuktu Triangle, the two military officers said.

Both officers asked not to be identified as they were not authorised to speak on the incident.

Nigerian military officials were not available for comment on the attack.

ISWAP split from the mainstream Boko Haram in 2016 to become the dominant militant faction in the northeast.

They seized territory hitherto under Boko Haram control, including the Timbuktu Triangle and Sambisa forest, a game reserve turned jihadist stronghold.

The group is notorious for planting roadside mines and rigging vehicles with explosives to target troops.

In July last year, seven troops were killed when their vehicle hit a landmine in a village where ISWAP is active.

The 15-year-old conflict has killed 40,000 and displaced around two million from their homes in the northeast.

The violence spilled into neighbouring Niger, Chad and Cameroon, leading to the creation of a regional force to fight the militants.