A bomb blast has hit a bus station in northeastern Nigeria killing at least 40 people, according to witnesses and security personnel.

At least five soldiers were among the victims of Thursday's attack in a village just outside of Mubi, the second largest city in Adamawa state.

No group has claimed responsibility for the attack but the area has been repeatedly targeted by the rebel group Boko Haram.

Al Jazeera's Rawya Rageh, reporting from Yola, said Mubi was a key town for Boko Haram and was one of the largest areas captured by the group after it launched its offensive in Adamawa State.

"Mubi has been the site of fierce clashes for weeks, with the military repeatedly claiming to have successfully taken the city back," Rageh said. 

In early November, Boko Haram seized the town, which has a population of more than 200,000, and renamed it "Madinat-ul Islam," or the city of Islam.

The regional government imposed a 24-hour curfew in Mubi to flush out any remaining Boko Haram fighters, Rageh said.

More than 1,500 people have been killed by the Boko Haram insurgency this year, according to Amnesty International.

Since May 2013, at least 1.5 million people have fled their homes since Nigeria declared a state of emergency in Adamawa, Yobe and Borno states, according to UN figures.

Source: Al Jazeera and agencies