A female suicide bomber blew herself up at a crowded market in Biu, northeast Nigeria, on Thursday, the latest in a spate of similar attacks by women and girls in the region besieged by Boko Haram.

At least seven people were killed and 20 injured, witnesses said Thursday.

"I counted seven people (dead) at the spot and more than 20 others are seriously bleeding. Shops were also affected by the... blast," said Zakka Emmanuel who was at the scene.

Trader Auwal Yusuf gave an identical toll and both said that the bomber may have been disguised as a passenger in a van loaded with goods and people.