The British oil giant said Wednesday would provide the first opportunity to launch the "top kill" procedure, which would smother a ruptured oil pipeline with heavy drilling fluid before ultimately sealing it with concrete.

But as Obama announced he would make a second trip to Louisiana to review the situation at first hand on Friday, BP conceded it was far from certain that the process would succeed.

"If it was on land we would have a very high confidence of success, but... we need to be realistic about the issues around operating in a mile of water," BP CEO Tony Hayward told reporters on Monday.

"We rate the probability of the success somewhere between 60 percent and 70 percent," he said.

More than a month after it began, the oil spill has leaked hundreds of thousands of gallons of crude into the sea, which is now lapping up on the shores of the Gulf of Mexico coast threatening rare animal and plant life.

Carol Browner, the top White House advisor on energy and environmental matters, said Tuesday it was the worst spill in US history.

"I don't think there's any doubt, unfortunately," she told ABC News.

As thick layers of oil sat on pristine beaches and lapped at reeds in environmentally-fragile marshlands, BP admitted it was not even sure when the capping effort would begin.

It was supposed to start early Wednesday, but BP senior vice president Kent Wells said Tuesday that a "diagnostic" assessment of the equipment "could take 12-24 hours."

"When the actual kill might go forward, the earliest might be tomorrow and that could extend on from there," Wells said.

Deputy Interior Secretary David Hayes told a US Senate panel that a decision would be made on Tuesday night or Wednesday morning about whether to launch the process, which involves pumping heavy drilling mud into the ruptured pipe at high enough pressure that the mud overcomes the flow of oil and gas.

Even as they were keeping their fingers crossed that the "top kill" would succeed, officials were readying other backup plans.

But some of the plans, including the drilling of relief wells to divert the flow and allow the original well to be capped, could take several months to be realized.

At the moment, if a "top kill" cannot proceed, BP plans to lower a containment dome over the leak, sealing it in place to prevent the formation of ice-like hydrates caused by cold sea water mixing with natural gas.

Such hydrates caused the failure of BP's first attempt to contain the oil leak by lowering a "top hat" onto it.

In Louisiana, Governor Bobby Jindal has called for the construction of berms or islands that could protect the coastline against advancing oil, but he has also acknowledged that it would take six to nine months to put them in place.

In Washington, Obama raised the oil spill in meetings with Republicans, telling them the disaster "should heighten our sense of urgency to hasten the development of new, clean energy sources," White House spokesman Robert Gibbs said.

Political fallout was mounting, with a new round of congressional hearings into the growing disaster, sparked by an explosion on a BP-leased Deepwater Horizon rig on April 20, which killed 11 workers.

The Minerals Management Service, which regulates offshore drilling, also came under new scrutiny after a Department of Interior report said staff members used illegal drugs, accepted gifts from oil companies and falsified inspection reports.

Wildlife and shorelines across four US states are threatened by the spill, though Louisiana has so far borne the brunt of the disaster, with thick oil washing into its fragile marshlands, coating wildlife in crude.

In Biloxi, Mississippi, fishmongers said they had been forced to import shrimps from Alabama.

"Most of the shrimpers are not allowed to go out," he told AFP. "A lot of clients... are asking if we've found oil in the shrimps."

Local residents and officials have expressed frustration at BP's failure to stop the leak, and with the pace of the clean-up efforts.

But BP executive Hayward pledged the oil giant would do everything in its power to cap the leak and clean up the damage.

"What I can tell you is that we are here for the long haul," the BP executive said. "We are going to clean every drop of oil off the shore."

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