Police in Nigeria say gunmen have kidnapped an American citizen in the oil-rich Niger Delta region.

Officials say the assailants abducted the man in the oil city of Warri Friday, killing his driver. The gunmen later demanded a ransom of more than $310,000.

The U.S. embassy in Nigeria said diplomats there were aware of the abduction.

The Niger Delta region produces billions of dollars in revenue for Nigeria's government, but is home to criminal gangs that steal oil and take hostages for ransom. Abductors typically target oil executives or oil workers.