A senior White House official has described the oil leak in the Gulf of Mexico as "probably the biggest environmental disaster" in US history.
White House energy adviser Carol Browner made her comments during an interview on NBC's "Meet the Press."
Ms Browner made her comments after BP said its attempt to plug the oil leak in the Gulf of Mexico had failed.
"There could be oil coming up till August when the relief wells are done", Ms Browner said.
She said BP's latest effort to try to capture and contain oil would not provide a permanent solution or prevent some oil escaping into the sea even if the maneuver succeeded.
"We are prepared for the worst. We have been prepared from the beginning," she added.
Department of Justice officials are part of an ongoing federal investigation into the April 20 rig explosion that triggered the spill, and the Obama administration has not ruled out the possibility of a criminal prosecution.
"I think without question if the word criminal should be used in terms of an environmental crime against our country, that what's going on in the Gulf of Mexico is going to qualify," Massachusetts Democrat Congressman Ed Markey said.
President Barack Obama, hit with a tide of criticism that he is not sufficiently in command of the largest oil spill in US history, has tried to lower expectations of a short-term fix.
On Saturday, US Coast Guard Admiral Mary Landry, standing by as a BP executive told the world that the tricky "top kill" procedure had failed, said they needed to manage people's expectations better for a crisis soon entering its seventh week.
Rightly so, because Landry believes the best solution to definitively plug the leak in the Gulf of Mexico is a relief well. BP is already drilling one, but it will not be ready for at least two more months.
By that time, it is hard to say how much damage the oil spill will have inflicted on the unique ecosystem of the Gulf coast, coveted for its seafood, fishing and tourism.
It could all be whipped further into marshland by what promises to be the most active Atlantic storm season since 2005, the year of Hurricane Katrina.
Louisianans still recovering from Katrina's devastation had a glimmer of hope that London-based energy giant's top kill procedure would be able to plug the ruptured well one mile (1.6 km) under water. But after 72 hours, that came to an end.
"Sure, we held out hope that it might," said Elizabeth Cook, an environmental activist from Gretna, Louisiana, who was visiting the fishing hub of Venice to see the spill response first hand. "But, you know, they really don't know how to address a spill like this that deep into the water."
BP Chief Executive Tony Hayward had predicted that despite risks, the top kill - the injection of heavy fluid into the well - had a 60 to 70 percent chance of success. He said he did not know why it failed to stop the gusher.
The latest failure is likely to drive his credibility lower, along with his company's market value, which has dropped by 25 per cent since the Deepwater Horizon rig exploded on April 20, killing 11 workers
The US government estimated last week that 12,000 to 19,000 barrels (504,000 to 798,000 gallons/1.9 million to 3 million litres) a day are leaking from the well, far above BP's figure of 5,000 barrels.
At that rate, the government now knows that the Gulf disaster has surpassed the 1989 Exxon Valdez spill in Alaskan waters.
BP has moved to its next option, a lower marine riser package cap to contain the oil and channel it to a drill ship on the surface - rather than plug the well. That will take about four days to place.
"We believe the LMRP cap has the capability to capture the great majority of it," said BP Chief Operating Officer Doug Suttles. "I don't want to say 100 per cent."
Mindful of the growing anger among Gulf Coast residents and others worried about the environmental catastrophe, both Hayward and Obama made a point of talking about people and their livelihoods after the top kill failure.
Hayward said in a statement that the containment cap "is the most effective way to minimize the impact of the oil leak on the Gulf ecosystem and the people of the region.
Obama's words were more emotional.
"Every day that this leak continues is an assault on the people of the Gulf Coast region, their livelihoods, and the natural bounty that belongs to all of us," he said, also in a statement.
"It is as enraging as it is heartbreaking."
Reuters