The Gulf of Mexico oil spill along the U.S. coast is the biggest environmental disaster that the country has ever faced, White House energy and climate change adviser Carol Browner said on Sunday.

Browner told NBC's "Meet the Press" that "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."

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

Browner's comments came after British energy giant BP announced that its "top kill" effort to help contain Gulf oil spill had failed.

BP said Saturday that its attempt to plug the oil leak in the Gulf of Mexico with mud and cement, or the so-called "top kill" operation, has failed and it will shift to a new plan to stop the prolonged environmental disaster.

Browner also said the administration is "prepared for the worst."

The Deepwater Horizon drilling rig, owned by Transocean and leased by BP, sank April 22 some 52 km off Venice, Louisiana, after burning for roughly 36 hours. The untapped wellhead continues gushing oil into the Gulf of Mexico. With the oil continuing to come ashore, criticism of the spill response mounted.