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Three female suicide bombers have carried out an attack that has killed at least 27 people in north-eastern Nigeria, officials say.

Dozens were injured when the women blew themselves up outside a refugee camp near Maiduguri in Borno state - a stronghold of the Islamist militant group Boko Haram.

There has been an upsurge in violence in the city in recent months.

Boko Haram has been fighting to set up an Islamic state since 2009.

A report published by US anti-terrorist researchers last week said it is the first insurgency in history to use more women suicide bombers than men.

Baba Kura, a member of a vigilante force set up to fight the jihadists, told AFP news agency on Tuesday that the first bomber blew herself up near the camp, triggering panic.

"People were trying to close their shops when two other female bombers triggered their explosives, causing most of the casualties," he said.

Last year, Nigeria's government said that Boko Haram had been defeated.

But correspondents say that the army is failing to stop the attacks, and people have continued to flee their homes in Borno State and poured into camps.