The Gulf of Mexico oil spill is the worst environmental disaster the US has faced, a senior official has said.
White House energy adviser Carol Browner also said the US was "prepared for the worst scenario" that the leak might not be stopped before August.
BP is to try a new tactic after its latest failure to halt the leak, but says there is no guarantee of success.
BP chief operating officer Doug Suttles said even if it worked it would only halt a majority of the spill.
At least 20 million gallons have now spilled into the Gulf of Mexico, affecting more than 70 miles (110km) of Louisiana's coastline.
Eleven rig workers died when the Deepwater Horizon exploded and sank nearly six weeks ago.
Well, this isn't very good news. BP has once again failed to stop the leak. I know that Americans must be getting very frustrated with this. My prayers are with those most affected, particularly on the Lousiana coastline.
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