HAVANA, Aug 30 (Reuters) - A ferociously powerful Hurricane Gustav entered the Gulf of Mexico on Saturday after passing over western Cuba as a Category 4 storm, the second highest on the five-step Saffir-Simpson scale of hurricane intensity, the Cuban weather service said.

"The eye (of the hurricane) is now completely over water and moving away from Cuba," said Maritza Ballester of the Meteorological Institute.

Gustav was aimed at offshore oil fields where the United States gets a large part of its domestic crude oil and natural gas, and also at New Orleans, the city swamped just three years ago by Hurricane Katrina. (Reporting by Esteban Israel; Editing by Michael Christie and Peter Cooney)

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