Endeavour space shuttle astronauts sped up a final spacewalk early Saturday as their comrades rushed to prepare to return to Earth to beat a huge hurricane looming in the Caribbean sea, NASA said.

Mission Specialist Dave Williams and Flight Engineer Clay Anderson moved out on their spacewalk, the fourth of the extended flight, nearly an hour early at 1317 GMT, the space agency said in a statement.

Inside the International Space Station preparations for the Endeavour's separation from the ISS were well underway, the statement read.

The scheduled four and a half hour spacewalk was to install a stand for the shuttle's robotic arm extension boom on the ISS truss structure.

The astronauts also planned to install an instrumentation system antenna and retrieve two containers used for experiments.

On Friday the National Aeronautic and Space Administration said they were working to move ahead the scheduled landing of the Endeavour Wednesday by 24 hours due to Hurricane Dean, which was moving westerly in the Caribbean toward the Gulf of Mexico and potentially could interfere with shuttle mission control in Houston, Texas.

Dean intensified late Friday to a category four storm, packing fearsome winds of up to 240 kilometers (150 miles) per hour, as it blasted toward Jamaica.

US space officials are concerned that if the hurricane veers toward the Texas coast, where the Johnson Space Center is located, it could affect management of the spacecraft's planned landing in Florida.

"We'd really like to protect an option to end the mission on Tuesday," LeRoy Cain, chairman of the mission management team, told reporters.

"The center will have to make a decision in the mid-Sunday to mid-Monday on whether or not to close," he added. "But the decision to land Endeavour early would have to be made well before that."

To prepare for an earlier return, Saturday's spacewalk was trimmed by 90 minutes so the crew could close the station and shuttle hatches by the end of the day, NASA said.

That would allow an undocking Sunday and plans for a landing Tuesday, before Dean threatens the Houston area.