At least seven people were killed and 20 injured when a female suicide bomber blew herself up at a busy market in the town of Biu, northeast Nigeria, 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.