Hurricane Katrina became a Category 3 storm on Saturday and may pound the southern US state of Louisiana next Monday, after it slashed south Florida on leaving seven people dead.

Local authorities asked residents of low-lying coastal communities on Saturday to get ready to evacuate to higher ground before the hurricane strengthens and takes a "possible direct hit"on southeast Louisiana.

"Board up your homes, make sure you have enough medicine, make sure the car has enough gas. Do all things you normally do for a hurricane but treat this one differently because it is pointed towards New Orleans," Mayor C. Ray Nagin said at a news conference.

The National Hurricane Center has posted a hurricane watch for the southeastern coast of Louisiana, including New Orleans. The center said Katrina was a Category 3 storm with 115 mph (184 kph) sustained wind Saturday, and it was likely to strengthen over the Gulf of Mexico.

Katrina, which was a Category 1 storm with 80 mph ( 128 kph) wind when it hit Florida on Thursday, strengthened into a Category2 hurricane as it moved offshore Friday.

The White House asked residents along Louisiana's southeastern coast on Saturday to heed authorities' advice to evacuate as Hurricane Katrina headed for landfall.

"We urge residents in the areas that could be impacted to follow the recommendations of local authorities," White House spokesman Scott McClellan said at Crawford, Texas, where PresidentGeorge W. Bush was on vacation.