HOUSTON (Reuters) - An oil well on the ocean floor beneath a drilling rig that exploded and sank into the Gulf of Mexico began spewing oil on Saturday, the U.S. Coast Guard said.

The well, 5,000 feet beneath the ocean surface, was leaking about 1,000 barrels per day of oil, a Coast Guard spokeswoman said, in what the agency called a “very serious spill.” Remote underwater vehicles detected oil leaking from the riser and drill pipe, the spokeswoman said.

“We are classifying this as a very serious spill and we are using all our resources to help contain it,” Coast Guard Petty Officer Connie Terrell said.

Transocean Ltd’s Deepwater Horizon sank on Thursday after burning since Tuesday following an explosion while finishing a well for BP Plc 42 miles off the Louisiana coast. The Coast Guard on Friday suspended a search for 11 missing workers from the rig, who are presumed dead.

BP has deployed an armada of ships and aircraft to contain the oil slick, which could threaten Louisiana’s fragile coastline if it is not contained. Cleanup operations are currently on hold due to stormy seas, Terrell said.

So far, the spill is not comparable with the Exxon Valdez disaster, which spilled about 11 million gallons (50 million liters) of oil into the Prince William Sound in Alaska when it ran aground in 1989. The Transocean well is spewing about 42,000 gallons (190,900 liters) of oil a day into the ocean, the Coast Guard estimates.

The explosion came almost three weeks after President Barack Obama unveiled plans for a limited expansion of U.S. offshore oil and gas drilling. The explosion did not affect U.S. oil markets.

The blast occurred about 10 p.m. CDT on Tuesday (0300 GMT Wednesday) as the rig was capping a discovery well pending production, company officials said. Some 115 of the 126 workers on board at the time of the explosion were rescued.

It was the worst oil rig disaster since 2001, when a rig operated by Petrobras off the Brazilian coast exploded and killed 11 workers. The Piper Alpha rig in the North Sea off Scotland exploded in 1988, killing 167.