Chevron Corp. says an offshore rig being run for its subsidiary near Nigeria's oil-rich southern delta has caught fire and officials are still trying to account for all the workers there.

Chevron spokesman Scott Walker said the fire started early Monday morning. He said the rig was just off the coast of the country's Niger Delta.

Walker said the company was still searching for all the workers onboard the rig at the time.

Nigeria is the fifth-largest crude oil exporter to the U.S. It pumps about 2.4 million barrels of crude oil a day. However, more than 50 years of oil production has seen environmental damage through delta's maze of muddy creeks and mangroves.