At least 20 people have been killed in a series of explosions at a bus station in the northern Nigerian city of Kano.Several buses were destroyed in the attack in the Sabon Gari district - which is home to many Christians from southern Nigeria.No group has said it was behind the blasts but Islamist militants from the Boko Haram group have attacked Kano in the past.It is the largest city in the mainly Muslim north of Nigeria.One eyewitness told the BBC that he had seen 20 bodies after Monday's blasts.Several witnesses told Reuters that one of the buses targeted was full when the explosion happened, and was completely destroyed.'Many people dead'"I ran for my dear life and managed to get out of the park after the second blast," one witness told the news agency. "Many people are lying dead."In January 2012, about 150 people died in Kano in a series of co-ordinated attacks by Boko Haram.The group is fighting to overthrow the Nigeria government and create an Islamic state.It is also believed to have a presence in Nigeria, Cameroon, Niger and Chad.Also on Monday, an audio tape emerged claiming to show one of seven French hostages captured by Boko Haram in neighbouring Cameroon last month.In the tape the group demands the release of its members it says are imprisoned in Nigeria and Cameroon in exchange for the hostages.Earlier this month, another militant Islamist organisation, Ansaru, claimed to have killed seven foreign hostages seized in the northern Nigerian state of Bauchi in February.