Kano - A series of blasts targeting three buses full of passengers in Nigeria's second city of Kano killed 20 people on Monday, a rescue official told reporters.

The Sabon Gari area of the city where the explosions occurred was cordoned off by soldiers after bloodied bystanders fled the scene in panic, according to residents.

Kano, the largest city in Nigeria's mainly Muslim north, has been repeatedly targeted by Islamist group Boko Haram, blamed for killing hundreds in the region since 2009.

“I saw three buses on fire. One of them was fully loaded with passengers waiting to leave the station at the time of the blasts. At the moment there are at least 20 dead,” said the rescue official who requested anonymity.

“This figure is not the final toll because we are still conducting rescue and the figure may rise,” added the official, who asked his name be withheld as he was not authorised to speak to journalists.

Mechanic Tunde Kazeem, who works at the targeted New Road bus station, said the explosion was “followed by billows of black smoke and there was a lot of confusion with people rushing out of the motor park, some of them with blood on their clothes”.

While the official confirmed at least two explosions, some residents have reported hearing up to three blasts, in what may have been a co-ordinated attack that is likely to blamed on Boko Haram.

The station was attacked in January of last year in a blast which wounded several people and which was blamed on the radical Islamist group.

The security forces were not immediately available to comment on the attack and there has not yet been a claim of responsibility.

The targeted station primarily services passengers heading to the mostly Christian south of Nigeria.

Boko Haram has previously targeted Christians, including through a series of suicide bombings at churches packed with worshippers on Sundays.

The group has also been blamed for killing officials, security personnel and other symbols of authority in an insurgency it says is aimed at establishing a Muslim state in northern Nigeria.

The group has also purportedly claimed the kidnapping of a seven members of a French family, including four children, abducted last month near in the Nigerian border in Cameroon.

In an audio recording obtained by AFP on Monday, Boko Haram demanded the release of its members it says are being held in Nigeria and Cameroon in exchange for the release of the hostages.

Boko Haram is believed to include a number of factions with various interests and shifting demands.

Its presumed leader, Abubakar Shekau, has been designated a global terrorist by the United States and experts say the group may have fostered ties with foreign extremists, including al-Qaeda's North Africa affiliate. - Sapa-AFP