BP says it has begun pumping mud into a breached Gulf of Mexico oil well to try to stem the flow of oil caused by a rig explosion last month.
Chief Executive Tony Hayward had earlier said the company would go ahead with the "top kill" measure, which has never been tried at such a depth.
BP is under intense pressure to succeed with its latest attempt to stem the leak, after previous measures failed.
Thousands of barrels of oil have been spewing into the Gulf every day.
The US government had earlier approved the move.
Company officials say it could be a couple of days before they know whether the top kill procedure is working.
Engineers hope to pump enough mud into the leaking well to subdue the oil flow, and then follow up with cement to permanently seal it.
Mr Hayward has put the top kill's chance of success at 60-70%.
Speaking in California, US President Barack Obama said his administration would commit all resources necessary to stop the flow of oil into the ocean.
"If it's successful – and there are no guarantees – it should greatly reduce or eliminate the flow of oil now streaming into the Gulf from the seafloor. And if it's not, there other approaches that may be viable."
Mr Obama said the "heartbreaking" oil spill underscored the need to find alternative energy sources.
"We will not rest until this well is shut, the environment is repaired and the clean-up is complete," he added.
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