KANO: A series of blasts targeting buses full of passengers in Nigeria's second city of Kano killed at least 20 people on Monday, but the toll was expected to rise, a rescue official and a security source told AFP.

Initial reports indicated that two suicide bombers rammed a car packed with explosives into a bus at the New Road station in Sabon Gari, a predominantly Christian neighbourhood in the majority Muslim city.

Several explosions were heard following the initial blast, sparking panic as bloodied bystanders including some with serious injuries fled the scene as soldiers arrived to cordon off the area.

Kano, the largest city in Nigeria's mainly Muslim north, has been repeatedly targeted by 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... There are at least 20 dead,” said the rescue official who requested anonymity as he was not authorised to speak to journalists.

“The figure may rise,” he added.

A senior security official in Kano, who also declined to be named, told AFP he believed the death toll was “massive”, describing the figure of 20 as an “understatement”, without giving a precise toll.

Fatima Abdullahi, 30, who had boarded a bus scheduled to head south, said she saw a car with two men inside ram into a nearby bus.

“There was a huge explosion followed by another. The bus went up in flames,” she told AFP at a hospital in Kano where she was being treated for her injuries.

Mechanic Tunde Kazeem, who works at the station, said he saw “people rushing out of the motor park” after the blasts, “some of them with blood on their clothes”.

The security source also said it was likely that suicide bombers carried out the attack, however details were still emerging and the cause had not yet been definitively confirmed.

There was also no immediate claim of responsibility, but the seemingly coordinated attack was likely to be blamed on Boko Haram.

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

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

That bombing came days after the group's deadliest ever attack, also in Kano, when at least 185 people were killed on January 20, 2012.

“Rescuers and security personnel are yet to determine the source of explosions...casualty figures not available at the moment, the seriously injured have been taken to hospital and bodies evacuated,” said a statement from the National Emergency Management Agency.

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 and experts say the group may have fostered ties with foreign extremists, including al Qaeda's north Africa affiliate.

Nigeria is Africa's most populous nation and largest oil producer, where poverty remains rampant, particularly in the north.