Crude oil is spewing from a sunken oil rig in the Gulf of Mexico, leaving a massive slick covering an area of 1,035 square kilometres, officials said yesterday.

An overflight detected a 20-mile by 20-mile slick emanating from the semi-submersible Deepwater Horizon platform that spectacularly sank last Thursday, two days after a massive blast left 11 workers missing and presumed dead.

British oil giant BP, which leases the platform, initially said no oil was leaking from the site but a robotic vessel on Saturday detected two holes in the riser that connects the wellhead to the sunken rig.

BP spokesman Ron Rybarczyk estimated that the leaks, almost one mile down on the ocean floor, were releasing oil out into the sea at the rate of 1,000 barrels, 42,000 gallons, a day.

"It is a serious spill," warned US Coast Guard spokesman petty officer Erik Swanson. "We are responding as if it is already a very serious spill but we're still assessing it."

BP and the owner of the rig, Houston-based contractor Transocean - already facing lawsuits from the families of those injured and presumed killed in the accident - are desperately hoping to avert an added environmental disaster. A coast guard plane located the slick on Saturday about 40 miles off the coast of Louisiana, home to a vast array of waterfowl and other wildlife. BP says it is considering two options to seal the leaks.

One would use a hydraulic device called a blowout preventer, installed near the wellhead as part of the existing equipment on the rig, to seal off the source of the oil.

A more time-consuming plan B is to set up what is called a relief well: this would involve drilling a new hole near the well, intercepting the leaking pipe, then pumping cement or heavy mud into the hole until it is fully sealed.

"We have had a visual image of the (sunken) rig," said Mr Rybarczyk. "We know that the rig is stable and it is staying in one place. Our approach now is to make the best fix toward getting those release points stopped."

There was uncertainty as to whether 700,000 gallons of diesel fuel on board the rig at the time of Tuesday night's explosion was also contributing to the pollution. "The disposition of the diesel fuel is unknown," BP spokesman Rybarczyk said. Thunderstorms and rough seas on Saturday hampered operations involving a flotilla of oil skimming vessels that had so far recovered less than 35,000 gallons of crude.

The massive operation includes seven skimming vessels, nine aircraft ready to deploy dispersants - seven fixed wing aircraft and two helicopters - and three barges for crude recovery.

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