WASHINGTON, Aug 29 (Reuters) - U.S. President George W. Bush on Friday declared an emergency in Louisiana as Tropical Storm Gustav headed for the Gulf of Mexico on the third anniversary of Hurricane Katrina's deadly strike on New Orleans.

"The president today declared an emergency exists in the State of Louisiana and ordered federal aid to supplement state and local response efforts due to the emergency conditions resulting from ... Gustav beginning on August 27, 2008, and continuing," the White House said in a statement.

Gustav was expected to strengthen in the warm Caribbean on Friday as it left flooded Jamaica and churned toward the Cayman Islands.

The storm, which killed at least 70 people in the Caribbean, plowed toward superheated waters south of Cuba where it could absorb enough energy to strengthen into a major hurricane before ripping through the heavy concentration of U.S. oil and natural gas platforms off Louisiana.

The White House says it is closely watching storm developments. Bush and his administration were widely criticized for a slow federal response to Katrina. (Reporting by Matt Spetalnick; editing by Mohammad Zargham)

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