Tuesday's first blast happened at the Tashan Dan Borno bus station, shortly before midday local time, on the outskirts of Potiskum in northeastern Nigeria.

Four hours later the Kano Line Station in Nigeria's largest northern city was rocked by a blast that sent people rushing from the scene.

At least 27 people were killed in the attacks, and dozens of injuries were reported.

Potiskum bus full of passengers

A driver's union official at Potiskum's bus station said the first bus targeted had "just loaded with passengers on its way to Kano."

Bus driver Adamu Isa said a man ignored a metal detector security check, forced himself onto the bus, and then "blew himself up."

"I was at a car wash nearby when I heard a huge explosion and saw volleys of fire and smoke coming from the motor park," said Waziri Danu, who lives in the area.

"I and people around [me] rushed to the place, and we saw a bus engulfed in flames."

Kano bus station shaken

In Kano, a local shopkeeper who was selling oranges to customers said the bus station was shaken by a blast. He then saw "plumes of black smoke."

"People and buses were rushing out of the bus station, " he said.

Kano has been among the locations hardest hit during a six-year insurgency by Nigeria's Boko Haram Islamist extremist group.

Using children as weapons

On Sunday, a girl aged seven or eight blew herself up at Potiskum market, killing five people.

The group has been accused of increasingly using young women and girls as suicide bombers, attacking "soft targets" such as bus stops and open markets.

Boko Haram rebels were recently confronted by troops from Nigeria, Niger, Cameroon and Chad.

dj,ipj/mg (AP, Reuters, AFP, dpa)