PORT HARCOURT, Nigeria - A gunman on a motorcycle shot dead a U.S. oil executive in an apparently planned assassination in Nigeria's oil heartland on Wednesday, authorities said.

Militants who have been waging a five-month-long campaign against the oil industry said they had no hand in the killing of the executive, who worked for Texan oil services company Baker Hughes. A diplomat and oil industry source said it was more likely to be linked to a work-related dispute.

"The American was shot by a man on a motorcycle. The motorcycle pulled up beside him and shot him," Samuel Agbetuyi, Rivers State Police Commissioner, told Reuters in the southern city of Port Harcourt, where the attack happened.

A spokesman for Baker Hughes, which drills wells and performs other services for major oil companies, confirmed an employee was shot on his way to work on Wednesday.