Seven suspected Boko Haram militants blew themselves on the outskirts of a northeast Nigerian city on Friday, a local aid agency said, in an attack witnesses said targeted refugees preparing to return to their home villages.

The bombing took place outside Maiduguri, the population centre at the heart of a government campaign to eradicate the Islamist group, whose more than seven-year insurgency has killed 15,000 people and forced some two million from their homes.

The Borno State Emergency Management Agency said eight members of a local militia, the civilian Joint Task Force, were wounded in the attack, which underscored Boko Haram's ability to continue to operate despite the government's insistence it has crushed the group.

Witnesses told Reuters the attackers detonated their bombs

near a large refugee camp, outside which crowds of displaced people were gathering around trucks to form convoys before trying to return home.

In December, President Muhammadu Buhari said the capture of a key camp marked the "final crushing" of Boko Haram in its last enclave in Sambisa forest, once the group's stronghold.

But since then the group, which split into two factions last year, has stepped up its attacks.

One Boko Haram faction is led by Abubakar Shekau from the Sambisa forest and the other, allied to jihadist group Islamic State, and led by Abu Musab al-Barnawi, based in the Lake Chad region.