Executives from the three firms tied to the April 20 oil rig explosion that killed 11 workers meanwhile blamed each other for setting off one of the worst oil spills in US history.

BP, which operated the Deepwater Horizon offshore oil platform before it exploded, caught fire and sank, said the rig's owner Transocean Ltd. was responsible for the failure of a key set of valves known as a blowout preventer that made it impossible to regain control of the well.

But Transocean instead blamed Halliburton, saying the oil services company was responsible for the well's cement casing, a temporary cement plug in the top of the well, and the cement's integrity.

The competing claims were made in prepared testimony by company executives to the Senate Energy and Natural Resources Committee, which kicks off a series of hearings on offshore oil drilling safety as the massive slick threatens the environment and coastal communities' livelihoods.

After failing to contain the spill with a giant dome over the weekend, BP said Monday it would make a second attempt this week using a smaller version dubbed the "top hat".

A four-story, 100-ton box was lowered Friday to the seabed nearly a mile (1,500 meters) below the surface to try to capture most of the oil and allow it to be funneled up to a ship above.

But it was rendered useless on Sunday when ice crystals formed in its domed roof.

BP experts believe the smaller version would not hold as much freezing cold seawater in the inky depths and thus avert a repeat of the problem.

The company's chief executive Tony Hayward told reporters he hoped the smaller container would be in place by Thursday and officials said it should be up and running this week.

Hayward admitted the smaller size meant it "will be less efficient at capturing" the leaking oil than the larger dome, which had been expected to swallow up to 85 percent of the crude.

With BP under intense pressure to plug the leak from a fractured pipe on the seabed, Hayward stressed the disaster was the company's first "major accident" in 20 years.

"We are taking this incredibly seriously" and responding in a "very, very aggressive way," he told National Public Radio.

BP said its disaster-related costs have reached 350 million dollars since the Deepwater Horizon rig sank 50 miles (80 kilometers) off the Louisiana coast on April 22, two days after the explosion.

The ruptured well is now spewing an estimated 210,000 gallons of crude into the sea each day.

With some 3.8 million gallons already in the sea causing untold damage to the fragile Gulf Coast, engineers are searching furiously for a quicker solution than a relief well that may not be ready until August.

Obama huddled with cabinet members in the secure Situation Room on Monday "to decide on next steps" in efforts to contain a steadily expanding disaster, and to keep the pressure squarely on BP to "aggressively" pursue solutions to stem the massive leak, the White House said.

BP is also preparing for a bizarre operation to inject golf balls, tires and other "junk" into the main leak, and then jam it up.

The "junk shot" could be risky as experts have warned that tinkering with the blowout preventer could see crude shoot out unchecked at 12 times the current rate.

BP also resumed streaming dispersants directly into the leak despite fears the chemicals themselves could be harming aquatic life from the smallest microorganisms on up.

The US Navy bolstered its role in skimming and salvage operations, including with 16 Modular Skimming Systems deployed to Gulfport, Mississippi. A total of 1,400 military personnel have been deployed to support cleanup and mitigation, officials said.

Sheen from the leading edge of the slick has surrounded island nature reserves off Louisiana's coast and tar balls have reached as far as the Alabama coast, threatening tourist beaches further east.

Sea life has been affected in a region that is a major migratory spot for rare birds and contains vital spawning grounds for fish, shrimp and crabs.

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