Islamic State-allied jihadists killed 18 Nigerian security personnel in an ambush on their convoy using roadside bombs and rockets in one of the deadliest assaults on the army this year, military sources said on Thursday.
Islamic State West Africa Province (ISWAP) militants claimed responsibility for Wednesday's attack on a highway between northeast Borno State capital Maiduguri and the garrison town of Monguno.
Borno is at the heart of Nigeria's grinding 12-year jihadist insurgency that has killed more than 40,000 people, displaced two million more and spilled over borders into neighbouring Chad, Cameroon and Niger.
Jihadists detonated roadside bombs on Wednesday before opening fire with rocket-propelled grenades and heavy weaponry, killing 16 soldiers and two members of a militia that helps fight ISWAP, the military sources said.
"Sixteen soldiers and two members of the Civilian JTF (militia) paid the supreme price in the attack while two soldiers were taken hostage by the terrorists," one military officer said.
Eleven troops were also wounded in Wednesday's attack in which several military vehicles were destroyed, said the officer who asked not to be identified as he was not authorised to speak to the media.
An army spokesman did not immediately answer calls seeking comment.
Another military source said the jihadists had detonated seven improvised explosive devices planted in the road as the convoy drove past. He gave a similar casualty toll.
ISWAP on Wednesday claimed responsibility for the attack, saying its fighters killed 25 troops in an ambush on a convoy, according to SITE, which monitors jihadist activities.
Nigeria's air force also said on Thursday it was investigating reports air strikes against jihadists in neighbouring northeast Yobe State had accidently killed and wounded civilians.
Residents in Buhari village said ten people had been killed and at least 19 wounded by a jet strike. The air force did not confirm any casualty figure.
Wednesday's highway ambush was the most high-profile since ISWAP began consolidating its position in Nigeria's northeast following the death of rival Boko Haram commander Abubakar Shekau in May.
Shekau died during infighting between his men and ISWAP militants in his Sambisa forest enclave, which has fallen under ISWAP control.
ISWAP has recently intensified attacks on civilians along the 135-kilometre (84-mile) Maiduguri-Monguno highway where they set up checkpoints, robbing and killing motorists, according to accounts of local residents.
The near daily attacks prompted military patrols along the highway, the military sources said.
Since 2019, soldiers have mostly shut down smaller army bases and moved into larger, fortified garrisons known as "super camps" in an attempt to better resist militant attacks.
But critics say the "super camp" strategy has also allowed jihadists more liberty to move around in rural areas and left travellers more vulnerable to kidnapping along highways.
Jihadist violence in the northeast is just one security challenge facing President Muhammadu Buhari as his armed forces also battle heavily armed criminal gangs behind mass abductions and looting in the northwest.