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The Deepwater Horizon oil rig burns on Wednesday after an explosion the night before. The blast happened as the rig was doing exploratory drilling in the Gulf of Mexico more than 80 kilometres off the Louisiana shore, ((Gerald Herbert/Associated Press))

Rescuers in helicopters and boats searched the Gulf of Mexico for 11 missing oil workers Wednesday after a thunderous explosion at a huge drilling platform that lit up the night sky with a pillar of flame.

Seventeen people were injured, four critically, in the blast Tuesday night aboard the Deepwater Horizon rig 80 kilometres off the Louisiana coast.

The blast could prove to be one of the deadliest U.S. offshore drilling accidents of the past half-century.

The U.S. coast guard held out hope that the missing workers escaped in one of the platform's covered lifeboats.

Nearly 24 hours after the explosion, the roughly 120-by-75-metre rig continued to burn, and authorities could not say when the flames might die out. A column of black smoke rose hundreds of metres over the Gulf of Mexico as fireboats shot streams of water at the blaze.

"We're hoping everyone's in a life raft," coast guard Senior Chief Petty Officer Mike O'Berry said.

Adrian Rose, vice-president of rig owner Transocean Ltd., said the explosion appeared to be a blowout, in which natural gas or oil forces its way up a well pipe and smashes the equipment. But precisely what went wrong is still under investigation.

Crews were doing routine work before the explosion, and there were no signs of trouble, Rose said.

A total of 126 workers were aboard the rig when it blew up. The coast guard said 17 were taken by air or sea to hospitals, including four in critical condition. Others suffered burns, broken legs and smoke inhalation.

Nearly 100 other workers made it aboard a supply boat and were expected to reach the Louisiana shore by evening.

Kelly Eugene waited with nine family members for her husband, Kevin Eugene, 46, a cook on the Deepwater Horizon. A catering company operating on the rig notified her he was safe.

"He's on the boat," she said. "That's all we know, and that's all we need to know."

The rig was tilting as much as 10 degrees after the blast, but earlier fears that it might topple over appeared unfounded. Coast guard environmental teams were on standby, though officials said the damage to the environment appeared minimal so far.

The rig, which was under contract to the oil giant BP, was doing exploratory drilling but was not in production, Transocean spokesman Greg Panagos said. Among the people on board, 79 worked for Transocean, six for BP and 41 were contract workers.

According to Transocean's website, the Deepwater Horizon is about twice the size of a U.S. football field. Built in 2001 in South Korea, it is designed to operate in water up to 2,500 metres deep, drill nearly nine kilometres down and accommodate a crew of 130. It floats on pontoons and is moored to the sea floor by several large anchors.