The storm has a 20 percent chance of reaching hurricane speeds of 74-mph (119-kph) before it makes landfall on Tuesday morning near Galveston, Texas, the Miami-based center said.

The storm, which formed near a major oil- and gas-producing area of the northern Gulf of Mexico on Sunday, was about 160 miles (260 km) south-southeast of Lafayette, Louisiana, and 265 miles (425 km) east-southeast of Galveston. It was moving west at about 8 mph (13 kmh).

A tropical storm warning was in effect from the mouth of the Mississippi River in Louisiana to San Luis Pass, south of Galveston.

Edouard, the second named storm to threaten oil operations in the Gulf of Mexico so far this year, shut down a huge offshore oil port, closed the Houston Ship Channel, and prompted Chevron Corp and Shell Oil to evacuate staff from their offshore platforms.

But so far, energy companies reported no production slowdowns as a result of the foul weather. The Gulf of Mexico supplies about a quarter of the nation's crude oil and 15 percent of its natural gas, while refiners along the coast produce about a quarter of domestic gasoline.

US crude oil futures were down about US$1 to US$124 a barrel by late morning.

The Louisiana Offshore Oil Port, the only deep-water US oil port and a major conduit for the country's crude oil imports, said it temporarily suspended offloading oil tankers in the Gulf of Mexico due to high waves and winds.

A series of powerful hurricanes in 2004 and 2005, including Hurricane Katrina, toppled oil rigs and severed pipelines in the Gulf.

The six-month hurricane season, which began on June 1, has already seen two of its four storms strengthen into hurricanes. Last month was the third most active month of July for storms since Atlantic hurricane season records began in 1851.

The early and unusually vigorous activity has given storm experts reason to believe that predictions for an above average season could turn out to be accurate.

Among the storms this year, Hurricane Dolly came ashore on the southern Texas coast on July 23, dousing the area with tremendous downpours but causing relatively little damage.

Hurricane Bertha grazed Bermuda and became the eighth longest-lived Atlantic storm on record before fading over the cool waters of the northern Atlantic, while Tropical Storm Cristobal brought heavy rain to the Carolinas. (Additional reporting by Michael Christie in Miami and Erwin Seba in Houston; Editing by Bill Trott)