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Two female suicide bombers have attacked a mosque in the north-eastern Nigerian city of Maiduguri, killing 22 worshippers, emergency officials say.

The first bomber struck inside the mosque, while the second blew herself up outside as survivors tried to flee, eye witnesses told the BBC.

Seventeen other people were wounded in the attack, an official told AFP.

Militant Islamists Boko Haram have often targeted the city in their seven-year insurgency.