"We have confirmed at least 20 rigs or platforms missing, either sunk or adrift, and one confirmed fire where a rig was," Petty Officer Robert Reed of the Louisiana Coast Guard told AFP.

All of the missing rigs were in the Gulf of Mexico, Reed said citing Coast Guard overflights of the area and information from oil companies.

He could not confirm the location of the blaze but said it would "eventually burn out" and no fire-fighting intervention was needed.

"We are of course working on the environmental side of things but right now we are still concentrating on search-and-rescue missions to save as many lives as possible on land," said Reed, whose own Coast Guard team has been evacuated from the flooded city of New Orleans to Alexandria, Louisiana.

According to the latest tally Wednesday from the federal Minerals Management Service, a total of 561 platforms and rigs have been evacuated in the Gulf of Mexico, which accounts for a quarter of US oil production.

Over 91 percent of normal daily crude oil production in the Gulf -- 1.5 million barrels -- is now shut down, and more than 83 percent of natural gas production, the MMS said.

Among the firms reporting missing rigs was Newfield Exploration Company, which said an aerial survey of its operations in the eastern Gulf showed that one of its platforms at Main Pass 138 "appears to have been lost in the storm".

"As of this morning, boats and helicopters are mobilizing to better access damages, identify any environmental impacts and begin the repair process," Newfield said in a statement.

Noble Corp. said its semi-submersible rig Noble Jim Thompson, which was contracted to a unit of Anglo-Dutch giant Shell, had broken loose and was 17 miles (27 kilometers) adrift of its normal Gulf location.

Shell's giant Mars platform was said by analysts to have suffered "extensive damage".

"Some of our facilities have been damaged and production is halted while we begin assessments and run checks," Shell chief executive Jeroen van der Veer said.

On shore, at least eight refineries have been shut down on the coasts of Louisiana and Mississippi since Katrina roared ashore early Monday just east of New Orleans.

Many other refineries are struggling to cope with shortfalls of crude caused by the closure of major port terminals and pipelines from evacuated and missing rigs in the Gulf of Mexico.

In response, the US government prepared Wednesday to open its emergency oil reserves for the first time in a year to keep supplies running to those refineries still operating.

The United States keeps 700 million barrels of oil stored in four underground salt caverns on the Texas and Louisiana coasts to cushion oil markets during supply disruptions.

The Strategic Petroleum Reserve, which was created during the 1970s oil shocks, was last used after Hurricane Ivan in September 2004, when five million barrels of crude oil were released.

"It's really getting critical in some situations," PFC Energy analyst Seth Kleinman said.

"Until refiners are up and running, and pipelines have power and get product up here, it's looking very precarious," he said.

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