Tropical Storm Frances, which developed sluggishly in the southern Gulf earlier this week, looked a lot more threatening by late Thursday afternoon.

Fuelled by the Gulf's warm waters, Frances gathered strength, with sustained wind speeds reaching 60 miles per hour (96 kph), raising the prospect that it could become a hurricane on Friday.

Tropical-storm-force winds of at least 39 miles per hour (62 kilometres) extended 345 miles (555 kilometres) across the Gulf with storm warnings posted from Tampico, Mexico to the Pearl River in Mississippi.

Thousands of offshore workers were evacuated by helicopter and companies suspended or "shut in" production at their deserted offshore oil and gas platforms until the storm passed.

It was only a week ago that Hurricane Earl swept through the Gulf of Mexico and across the Florida Panhandle,briefly knocking out three-quarters of Gulf oil and gas output on its way.

The rich oilfields off the Texas and Louisiana coasts usually produce more than 14 billion cubic feet of gas and over a million barrels of oil a day -- roughly one-quarter of US domestic gas production and one-sixth of the nation's oil output.

Details about the impact of the latest storm on oil and gas output were slow to emerge, but there were pointers that Frances could turn out to be just as disruptive as Earl.

Official figures released by the US Minerals Management Service (MMS) on Thursday afternoon suggested that less than 5 per cent of oil and gas production had been suspended.

But the MMS figures were rapidly overtaken by announcements from two of the Gulf's biggest oil and gas producers.

Amoco Corp said it had shut in 280 million cubic feet a day of natural gas -- just under half of its total daily output of gas in the Gulf of Mexico.

It also said the Amoco-operated High Island Pipeline System off the Texas coastwas running at only about 60 per cent of its normal capacity of 50,000 barrels of crude oil per day.

Chevron Corp said it had suspended 320 million cubic feet a day of its Gulf gas production, roughly one-third of its total daily gas output in the Gulf.

Spokeswoman Julia Fremin said Chevron had also suspended 15,000 barrels a day of oil production out of its total daily output in the Gulf of just over 100,000 barrels.

Fremin said Chevron had evacuated about 500 offshore workers, mainly from platforms in the western Gulf, and was continuing to monitor the storm to see if further measures would be needed.

"Right now, we're still in a holding pattern," she said. "We're watching everything very carefully."

Texaco Inc on Thursday evacuated 118 offshore oil workers from Gulf of Mexico drilling rigs and platforms as Frances moved through the area, said spokeswoman Beth Picou. Texaco hopes to have the staff back in place on Friday, she added, saying it was unclear whether oil and gas production had beenaffected.

And Exxon Corp also pulled employees and shut in production on platforms in the Corpus Christi, Texas, area, in response to the growing storm.

Strong winds and choppy seas forced the Louisiana Offshore Oil Port (LOOP) to stop unloading giant crude oil tankers for the second time in the last two weeks.

"The weather is simply too rough to moor ships...We've got seas that are 12 feet," LOOP official Mark Bugg said.

Located 20 miles off the Louisiana coast, LOOP is the only deep water oil Port in the United States, usually unloading 25 to 30 tankers a month with an average cargo of 1.1 million barrels of crude.

Tropical Storm Frances also closed the Houston Ship Channel, a busy maritime artery linking the Port city to the Gulf of Mexico.

A US Coast Guard spokesman said pilots had stopped boarding vessels seeking to enter the channel because strong winds and high waves had made it unsafe for them to do so.

The Houston Ship Channel provides access to industrial zones outside the city wherefour oil refineries are located.

On the New York Mercantile Exchange, crude oil and natural gas futures rose sharply, driven in part by the growing intensity of Tropical Storm Frances. The Nymex October crude oil futures contract settled at $14.67 a barrel, up 55 cents from Wednesday's close. The Nymex October gas futures contract rallied 12.5 cents to close at $1.958 per million British thermal units.

On Thursday, Chevron's stock fell 25 cents to $80.00 a share; Amoco's stock rose 28 cents to $49.84; Texaco's stock added 69 cents to $58.81, and Exxon's stock gained $1.00 to $69.69 -- all in composite New York Stock Exchange trade.