The swelling slick, now estimated to be at least 130 miles by 70 miles, or about the size of the state of Delaware, threatens shipping, wildlife, beaches and one of the United States' most fertile fishing grounds.
"This spill, it can fundamentally change our way of life here," Louisiana Gov. Bobby Jindal said.
BP, the British energy company, has been working to plug a leak nearly a mile under the surface of the ocean, under heavy pressure from the U.S. government to try to limit a looming ecological and economic disaster.
Tony Hayward, BP's chief executive, and Lamar McKay, BP America president, met on Monday with top Obama administration officials including the energy, interior and homeland security secretaries and the head of the Environmental Protection Agency, to discuss coordinated response efforts.
Separately, BP said crews in Louisiana have finished building the first of three massive steel and concrete containment domes the company plans to lower in place over one of the three leaks on the ocean floor.
"We will load that on a ship tomorrow along with other associated equipment, and transport it to the site," Doug Suttles, chief operating officer of BP's exploration and production unit, told reporters on a conference call.
Drilling also started Sunday night on a relief well that could cap the oil spill on the Gulf floor, the company said. Still, this operation is expected to take two to three months to complete.
THREATENS FOUR STATES
The National Oceanic and Atmospheric Administration projects that the oil slick, which continues to spread over a wider surface area in the warm Gulf waters, will move further east and west by Tuesday, although not necessarily further north toward the coast.
Efforts to prevent the slow-moving mass from washing ashore in parts of four states have been hampered for days by choppy seas and high waves in the Gulf, but forecasts suggest calmer conditions in the next few days.
"The stormy weather is clearing as we speak," Shawn O'Neil, a meteorologist with the National Weather Service in New Orleans, said. "The winds will stay light and variable all the way through Friday. They will have much improved conditions to do what they need to get done."
Miles of booms are being laid along the coast of four states in an effort to contain the movement of oil onto beaches and into key wildlife sanctuaries and breeding grounds.
The Obama administration has kept the focus on BP to pay for and assume responsibility for the oil spill disaster, which started with an explosion April 20 on the Deepwater Horizon rig that killed 11 workers.
For its part, the federal government has come under fire for not responding more quickly to the spreading economic and environmental threat -- criticisms that may have prompted Obama to travel to the affected region on Sunday.
The oil spill, which continues unchecked for now, could ultimately rival the Exxon Valdez disaster from 1989.
That spill was caused when a single, massive oil tanker spewed some 10.8 million gallons or 250,000 barrels of crude oil into Alaska's Prince William Sound.
Estimates put the daily flow from the current well leak at 5,000 barrels or more, but government and company officials have suggested they that estimate could be low.
Comments by U.S. Attorney General Eric Holder that the Justice Department was involved in the investigation of the incident raised the specter of criminal liability for BP over the spill.
A Justice Department official said it was not a criminal probe at this stage.
BP's Hayward on Monday acknowledged his company's responsibility in a round of appearances on American TV and radio shows, a day after U.S. Interior Secretary Ken Salazar said the administration would keep "keep the boot on the neck" of BP to fulfill its legal responsibilities.
Analysts say BP's total liability could exceed $14 billion.
In New York, American Depositary Receipts of BP's shares fell 3.7 percent to close at $50.19. Shares of Transocean Ltd, operator of the sunken oil rig, rose 0.82 percent to 72.91.
Argus Research on Monday downgraded both Transocean and BP to "hold" from "buy" citing negative impact from the Deepwater Horizon spill.
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