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WASHINGTON, Sept 5 (Reuters) - Hurricane Ike is shifting course and could strike the Florida Keys before moving into the oil-producing Gulf of Mexico, where it could strengthen, a U.S. emergency official said on Friday.

"That's kind of a bad scenario for us, to go into the Gulf," Federal Emergency Management Agency Assistant Administrator Glenn Cannon said. Moving over warm Gulf water could make the already powerful storm more dangerous.

Ike is a Category 3 hurricane on the five-step Saffir-Simpson scale. The storm's path was taking a more southerly direction and "changing as we talk," Cannon said.

He said the current course could cause it to "heavily impact" the Florida Keys.

Cannon, briefing reporters on a string of Atlantic storms, also said residents evacuated by federal officials last week due to Hurricane Gustav were expected to return home by the end of this weekend.

Mandatory evacuations due to Tropical Storm Hanna along the U.S. Southeast coast were expected to be minimal, he said.

He said 685,000 homes remained without power after Gustav's strike on Louisiana, and the continued outages made it hard for residents to obtain groceries and gasoline.

(Reporting by Randall Mikkelsen)

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