KEY WEST, Fla. – Rapidly strengthening Hurricane Rita lashed the Florida Keys yesterday and headed into the Gulf of Mexico, where forecasters feared it could develop into another blockbuster storm targeting Texas or Louisiana.

Thousands of people were evacuated from the Keys and low-lying areas of northern Cuba. On the far side of the Gulf of Mexico in Texas, Galveston started evacuations and officials made plans to move refugees from Hurricane Katrina who had been housed in the Houston area to Arkansas.

Forecasters said Rita could intensify in the Gulf of Mexico into a Category 4 storm with winds of at least 131 mph. The most likely destination by week’s end was Texas, although Louisiana and northern Mexico were possibilities, according to the hurricane center.

The acting FEMA director, R. David Paulison, told reporters that the agency has aircraft and buses available to evacuate residents of areas the hurricane might hit. Rescue teams and truckloads of ice, water, and prepared meals were being sent to Texas and Florida.

“I strongly urge Gulf Coast residents to pay attention” to the storm, he said.

Meanwhile, the Army Corps of Engineers raced to patch New Orleans’s fractured levee system yesterday, and residents were forced to decide yet again whether to stay or go as a new, rapidly strengthening hurricane threatened to flood the city anew.

“The protection is very tenuous at best,” said Dave Wurtzel, the Army Corps official responsible for repairing the 17th Street Canal levee, whose huge breach during Katrina caused the worst of the floods that wrecked the city.

Government engineers and private contractors also worked around the clock across New Orleans to repair the damage to the system of pumps, concrete floodwalls, earthen berms, and canals that protect the below-sea-level city.

In addition, the corps had 800 giant sandbags weighing 6,000 to 15,000 pounds on hand just in case, and ordered 2,500 more to shore up low spots and plug any new breaches. It was also putting pumps and other materials where they might be needed.

“If New Orleans was directly affected by a Category 1, I would be concerned – I would pull my people out,” said David Pezza, the top geotechnical engineer for the Army Corps. “These levees are greatly compromised.”

Stung by criticism of the government’s slow initial response to Hurricane Katrina, President Bush signed an emergency declaration for Florida and spoke with Governor Perry of Texas about planning for the storm’s landfall.

“All up and down the coastline people are now preparing for what is anticipated to be another significant storm,” Mr. Bush said.

Governor Bush of Florida said more than 2,000 Florida National Guard troops and dozens of law enforcement officers were ready to deal with the storm’s aftermath, although it appeared the Keys were spared the storm’s full fury.

“I think we did, so far, dodge a bullet,” said the mayor of Key West, Jimmy Weekley.

Rita started the day as a tropical storm with top sustained wind of 70 mph. But as it cruised through the Florida Straits between the Keys and Cuba, it gathered energy from the warm sea and by early afternoon it had top wind of 100 mph with higher gusts, the National Hurricane Center said.

President Bush received a briefing about Rita aboard the USS Iwo Jima, which is docked near downtown New Orleans, as the hurricane caused new anxiety among Katrina victims in Mississippi, Louisiana, and Alabama.

“There’s still plenty of warm water that it needs to move over in the next couple days. The forecast is favorable for further intensification,” said a meteorologist at the National Hurricane Center, Michelle Mainelli.

Residents and visitors had been ordered out of the Keys, and voluntary evacuation orders were posted for coastal mainland areas such as Miami Beach. Some 58,000 people were evacuated in Cuba, on the southern side of the Florida Straits.

Many of Key West’s shops and bars were boarded up.

“This city was really very well prepared,” said Jim Gilleran, owner of the 801 Bar in the Old Town section of Key West. He kept his business open despite the heavy rain and a power outage.

At least one segment of the Keys highway, U.S. 1, was barricaded be cause of water and debris, the Florida Highway Patrol said. Wind-driven water was flowing across other sections of the two- and three-lane highway that connects the Keys.

At 5 p.m., Rita’s eye was about 50 miles south-southwest of Key West. The storm was moving west at 15 mph on a track that would keep the most destructive winds at sea, the center said.

Nearly 900 miles from Key West, officials of Galveston were already calling for voluntary evacuations, with mandatory evacuations to begin Wednesday. Governor Blanco of Louisiana urged everyone in the southwest part of the state to prepare to evacuate.