A top energy adviser to United States President Barack Obama is calling the Gulf of Mexico oil spill the worst environmental disaster the US has faced.
Speaking on NBC television, Carol Browner said the US was prepared for the worst scenario, following BP's latest failure to halt the catastrophic leak.
The company says its so-called "top kill" operation to blast waste material and heavy mud into the ruptured Deepwater Horizon well has failed.
It says that it will try a new tactic - using an underwater robot to saw off the leaking pipe and place a cap over it - but that it will take at least four days to set up and that there is no guarantee of success.
Even if it does work, the company says, it will only halt most of the leak, not all of it.
Latest failure heartbreaking - Obama
Millions of litres of heavy crude continue to gush into the Gulf of Mexico six weeks after an explosion killed 11 people and ruptured the well.
Ms Browner said the leak might continue for months. She said BP's latest effort will not provide a permanent solution, even if it succeeds.
Mr Obama himself says BP's latest failure to stop the leak is heartbreaking. He says the US government will continue to pursue all means of plugging the leak until two relief wells are completed in several months' time.
BP disputes claim of large plumes
Meanwhile, BP chief executive Tony Hayward is disputing claims by scientists that there are large undersea plumes from the leak.
Scientists from several universities are reporting plumes of what appears to be oil suspended in clouds that stretch for kilometres and reach hundreds of metres beneath the Gulf's surface.
Mr Hayward says that the oil is on the surface, and that the company's own sampling shows no evidence of it in the water column.
