Witnesses in Nigeria say two explosions near a military base in the central city of Kaduna caused an unknown number of casualties Tuesday.

Yushau Shuaib, a spokesman for Nigeria's National Emergency Management Agency, says it was not immediately clear what caused the blasts.

Initial reports say one of the explosions occurred at a base of the army's 1st Mechanized Division.

There was no immediate claim of responsibility.

The radical Islamic sect known as Boko Haram has been blamed for a surge in violent attacks against police, military and government facilities.

Kaduna is 200 kilometers south of Kano, where Boko Haram claimed responsibility for a wave of attacks that killed about 180 people last month.

The militant group has said it is fighting to impose strict Islamic law across Nigeria, but analysts say political disputes appeared to be involved in some attacks.

Frequent bomb blasts and shootings in the north blamed on Boko Haram have prompted Nigeria's government to increase security.

President Goodluck Jonathan has declared a state of emergency in some parts of the region and vowed to defeat the group.