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The U.S. Coast Guard discovered Saturday that oil is leaking from the damaged well underneath a massive rig that exploded this week off Louisiana's shore, while bad weather halted efforts to clean up the mess that threatens the area's fragile marine ecosystem.

For days, the coast guard has said no oil appeared to be escaping from the wellhead on the ocean floor. Rear Admiral Mary Landry said the leak was a new discovery but could have begun when the offshore platform sank on Thursday, two days after the initial explosion.

"We thought what we were dealing with as of yesterday was a surface residual [oil] from the mobile offshore drilling unit," Landry said. "In addition to that is oil emanating from the well. It is a big change from yesterday .… This is a very serious spill, absolutely."

Coast guard and company officials estimate that as much as 1,000 barrels of oil is leaking each day after studying information from remotely operated vehicles and the size of the oil slick surrounding the blast site. The rainbow-coloured sheen of oil stretched 32 kilometres square on Saturday — about 25 times larger than it appeared to be a day earlier, Landry said.

Eleven workers are still missing from the Deepwater Horizon rig, which sank Thursday about 80 kilometres from Louisiana's coast. They are presumed dead, and the search for them was called off Friday.

Petroleum giant BP, which leased the rig and is taking the lead in the cleanup, and the government have been using remotely operated vehicles to try to stop the leak by closing valves on the well deep underwater. If that doesn't work, the company could drill what's called an intervention well to control the oil flow. But the intervention drilling could take months.

So far, crews have retrieved about 1,052 barrels of oily water. The sunken rig may have as much as 700,000 gallons of diesel on board, and an undetermined amount of oil has spilled from the rig itself.

BP said it has activated an extensive oil spill response, including the remotely operated vehicles, 700 workers, four airplanes and 32 vessels to mop up the spill. The Marine Spill Response Corp., an energy industry cleanup consortium, also brought equipment.

The 11 missing workers came from Texas, Louisiana and Mississippi. Neither the coast guard nor their employers have released their names, though several of their families have come forward.

The other 115 crew members made it off the platform; several were hurt but only one remained hospitalized. The most seriously injured worker was expected to be released within about 10 days.

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In this aerial photo taken in the Gulf of Mexico, the Deepwater Horizon oil rig is seen burning Wednesday. ((Gerald Herbert/Associated Press))