DAMATURU, Nigeria (Reuters) - A teenage girl blew herself up at a crowded bus station in a north-eastern Nigerian town on Tuesday, killing at least 15 people and wounding 53 in the second such attack there this week.

No one claimed responsibility for the bombing in Potiskum but the main suspect is likely to be extremist group Boko Haram, whose struggle for an Islamic state in religiously mixed Nigeria has killed thousands of people and displaced over a million.

Witness Musa Ayuba, who was knocked over by the force of the blast, said the girl arrived at the Tashan Dan Borno bus station in a rickshaw and was trying to board a bus when she detonated the bomb.

A security source confirmed the blast and a source at the local hospital said 15 bodies had so far been brought in and 53 people were receiving treatment for wounds, some serious.

On Sunday, a girl with explosives strapped to her killed five people and wounded dozens outside a market in Potiskum.

The use of female suicide bombers has become a common tactic of Boko Haram since last year as the group expanded territory and became stronger and more deadly. But in the past three weeks it has begun to suffer a string of defeats in a military offensive by Nigeria and neighbours Cameroon, Niger and Chad.

Last Saturday, Nigerian troops backed by air strikes seized the north-eastern border town of Baga from Boko Haram, the military said, a significant victory in the offensive.

But failure to protect civilians is a major criticism of President Goodluck Jonathan's administration ahead of an election scheduled for March 28.

Potiskum lies in Yobe state, which has seen many Boko Haram attacks but has not been taken over by them the way neighbouring Borno state has, and is hundreds of kilometres from the main theatre of the war against the insurgents.

On Monday, defence spokesman Major-General Chris Olukolade said air strikes were under way on Boko Haram targets in the Borno state towns of Gwoza, where the group first declared an Islamic state, Bama and Sambisa.