An explosion thought to have been caused by a gas cylinder has rocked a Central Bank of Nigeria building in the southern city of Calabar, killing at least two people and injuring 18 others, police said on Friday.

Nigeria is on high alert for militant attacks as Boko Haram fighters – who have killed thousands and displaced two million people in a seven-year-old insurgency in the northeast – may start targeting other parts of the country.

Cross River state police spokesman, John Elu, said the blast was thought to have occurred at about 11.30 a.m. (1030 GMT).

“The explosion is suspected to have been caused by a gas cylinder controlling the central air conditioner in the banking hall causing shock waves that affected other air conditioners there,” he said.

Acting Head of Corporate Communications of CBN, Isaac Okoroafor said it was a gas explosion. He added that there was no fatality. A few staff who sustained injuries had been treated, he said.

Additional Report: REUTERS