Suspected Boko Haram fighters have killed at least 24 members of a vigilante group in north-eastern Nigeria, security officials say.
They say another 34 people are missing after the attack near the town of Monguno in Borno state.
The officials said militants wearing army uniforms ambushed more than 100 vigilantes on Friday.
Attacks have increased recently despite a massive military deployment to areas worst affected by Islamist militants.
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Nigeria's army recently said it had killed Boko Haram's leader Abubakar Shekau but this has not been confirmed.
The military has encouraged the formation of vigilante groups to help tackle the Islamists.
The militants attacked the vigilante youths on the outskirts of Monguno, about 160km (100 miles) north-east of the state capital Maiduguri, where Boko Haram was formed in 2009.
The vigilante youths had been on a mission to capture Boko Haram militants in their camps when they were ambushed, the Sunday Tribune reports.
The vigilantes had originally arranged to go with the army but after some hours of waiting for the soldiers to arrive they moved in by themselves.
They were ambushed by militants who "had disguised in military uniforms with three captured patrol vehicles of the security agencies", sources told the paper.
There are fears that the death toll could rise further.
Boko Haram has waged a deadly insurgency in Nigeria since 2009, which has left several thousand dead.
In May, President Goodluck Jonathan declared an emergency in three north-eastern states, including Borno, saying the group threatened Nigeria's existence.
An offensive was launched against the group - which says it is fighting for the creation of an Islamic state in Nigeria - and the military encouraged the formation of vigilante groups to help.
But now it appears Boko Haram is taking revenge against such groups, say observers - adding weight to fears that the vigilante groups may trigger an escalation of the violence.
Last week, at least 20 vigilantes were killed in two separate attacks in Borno.