Oil spill 'biggest US environmental disaster'

The oil spill in the Gulf of Mexico is probably 'the biggest environmental disaster' that the US has ever experienced, a senior White House official says.

Carol Browner, a senior White House advisor on energy and climate change, told NBC the leak was "probably the biggest environmental disaster we've ever faced in this country".

Estimates now suggest that between 12,000 to 19,000 barrels of crude are gushing into the Gulf each day from a well pipe damaged when the Deepwater Horizon oil rig exploded and then sank two days later on April 22.

That means "more oil is leaking in the Gulf of Mexico than at any other time in our history. It means there is more oil than the Exxon Valdez," Browner said.

Approximately 11 million gallons of crude spilled into the sea during the 1989 Exxon Valdez disaster.

'Top kill' plan failed

Meanwhile, BP engineers are scrambling to implement another high-risk plan to stem the devastating spill, after their "top kill" plan failed.

"Right now we are going to a containment operation," BP managing director Bob Dudley told CNN - admitting that the latest project - placing a cap over the leak - could take some time.

"Because this is being done at 5000 feet (1524m) with robots, we're going to take our time, do it extremely carefully. By the end of the week, we should have this in place," Dudley said.

While the "top kill" plan would have sealed the well using a combination of heavy drilling fluid and eventually cement, the new effort aims only to reduce the leak, and might temporarily increase the amount of oil gushing into the Gulf of Mexico, officials said.

The well will only be sealed when BP finishes drilling two relief wells, but they are not expected to be ready until August.

At least 20 million gallons are now estimated to have leaked into the ocean since the Deepwater Horizon oil rig exploded on April 20 and sank, killing 11 workers.