Oil companies began evacuating workers from rigs in the Gulf of Mexico ahead of what US forecasters said could be a powerful and dangerous storm, and Louisiana declared a state of emergency.

The National Hurricane Center on Thursday issued tropical storm warnings for the coastal areas from Pascagoula, Mississippi, to Sabine Pass, Texas, even as areas of the northeast were recovering from record flooding from Hurricane Irene.

The new weather system, provisionally called Tropical Depression 13, could strengthen to a tropical storm, the hurricane center said, and hit areas of the coast where memories are still fresh of the devastation from Hurricane Katrina in 2005.

Louisiana Governor Bobby Jindal declared a state of emergency, saying the system poses "a threat of extremely heavy, prolonged rainfall resulting in very high tides for coastal parishes and the possibility of flash floods for coastal parishes and inland parishes through the Labor Day weekend."

He said some areas could receive receive up 12-15 inches (30 to 38 centimeters) of rainfall over 48 hours, based on current forecasts.

"We know from experience that it?s best to prepare for the worst and hope for the best, and since we?re at the height of the hurricane season, now is a good time for Louisianians to make sure they have a gameplan to protect themselves and their families if a major storm approaches our coast," Jindal said earlier.

The new disturbance was packing winds of 35 miles (55 kilometers) per hour and was centered about 225 miles (360 kilometers) south of the mouth of the Mississippi River, and moving north.

"Some strengthening is forecast during the next 48 hours and the depression could become a tropical storm on Friday," the hurricane center said.

"The depression is expected to move very slowly during the next five days, prolonging the period of tropical storm conditions and heavy rainfall over portions of the northern Gulf coast."

ExxonMobil said it was evacuating approximately 140 employees and contractors from Gulf Coast offshore platforms expected to be in the path of the storm.

"Gross production of approximately 11,000 barrels per day of liquids and 60 million cubic feet per day of natural gas has been shut-in," the oil giant said in a statement.

BP said it had begun evacuating "all personnel from its operative assets in the Gulf of Mexico" on Thursday, but did not indicate the impact on production.

Shell said weather conditions were interfering with evacuation efforts and it might be safer to leave some workers on their rigs.

"Our priorities at this time continue to be ensuring the safety of personnel, protecting the environment and minimizing production and operational impact," the Anglo-Dutch energy giant said in a statement.

Shell said it had begun to take actions to shut in some production, primarily in "subsea fields that require specific treatments to ensure the production can be restored after the storm passes," and that the impacts so far are "minimal."

Anadarko said it had shut in production at all eight of its facilities and was evacuating all personnel -- about 100 people.

Chevron said it was evacuating "non-essential personnel" and that production has not been affected.

The new storm approached as US President Barack Obama declared a "major" disaster area in New Jersey and announced plans to visit the flood-hit state on Sunday to view damage wrought by Hurricane Irene, the Atlantic season's first.

Thousands remain cut off by flooding in Vermont, New Jersey and upstate New York in the aftermath of Irene, which killed nearly 50 people.

Meanwhile, Katia was downgraded to a tropical storm Thursday but was expected to regain strength as it moved westward in the Atlantic, the National Hurricane Center said.

Katia was 930 miles (1495 kilometers) east of the northern Leeward Islands at 2100 GMT -- too far to affect any land mass -- and there was a chance it would cycle north and disintegrate without hitting the North American mainland.