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New Orleans, Louisiana - The estimated amount of oil leaking from a sunken rig in the Gulf of Mexico has increased to as much as 5,000 barrels a day -- five times more than what was originally believed, a Coast Guard official said.

Rear Adm. Mary Landry told reporters late Wednesday that the increased estimate is based on analysis from the federal National Oceanic and Atmospheric Administration.

"This is not an exact science when you estimate the amount of oil," Landry said, noting there are a lot of variables in calculating the rate of the spill.

"However, NOAA is telling me now that they prefer we use the 5,000 barrels [210,000 gallons] a day as an estimate of what has actually leaked from this well and will continue to leak until BP secures the source."

Some 250,000 gallons of oily water has been collected from the scene, she said.

BP is the owner of the well, while Transocean Ltd. owns and operates the rig.

Additionally, a third underwater oil leak has been located in the pipeline that connected the rig to the oil well, said Doug Suttles, chief operating officer for BP, who joined Landry at a news conference. Two other leaks were located within a few days of the April 20 explosion.

The cause of the explosion remains under investigation, and search efforts have been halted for 11 workers missing after the blast.

"I do not disagree with the admiral's estimate that it could be 5,000 barrels a day -- it's clearly within the range of uncertainty," Suttles said.

Landry said President Obama was briefed on the latest development from the spill site.

"The president urged out of an abundance of caution and, mindful of the new information, that BP must position resources to continue to aggressively confront this incident," Landry said. "That said, BP has always planned and anticipated for a much larger spill. We have urged BP to leverage additional assets to help lead the response to an oil spill incident for which they are responsible."

Military planners on Wednesday night began examining options to provide assistance to the Coast Guard in cleaning up the spill, said James Graybeal, a spokesman for U.S. Northern Command in Colorado Springs, Colorado. Northern Command is responsible for coordinating and providing military assistance inside the United States.

While the specifics of the assistance have yet to be determined, some of the ways the military could help would be putting a ship in the Gulf of Mexico to support and resupply other vessels in the region or providing aircraft to help map the spill.

The head of BP Group told CNN's Brian Todd in an exclusive interview Wednesday that the accident could have been prevented, and he focused blame on rig owner Transocean.

CEO Tony Hayward said that Transocean's blowout preventer failed to operate before the explosion. A blowout preventer is a large valve at the top of a well, and activating it will stop the flow of oil. The valve may be closed during drilling if underground pressure drives up oil or natural gas, threatening the rig.

"That is the ultimate fail-safe mechanism," Hayward said. "And for whatever reason -- and we don't understand that yet, but we clearly will as a consequence of both our investigation and federal investigations -- it failed to operate.

"And that is the key issue here, the failure of the Transocean BOP," Hayward said, describing the valve as "an integral part of the drilling rig"

A Transocean spokesman on Wednesday declined to respond to Hayward's comments in the CNN interview, citing pending litigation against both companies.

However, Transocean Vice President Adrian Rose has said its oil rig had no indication of problems before the explosion.

Asked whether the accident could have prevented, Hayward said, "All accidents can be prevented -- there's no doubt about that."

At least one of the victims' families has filed a lawsuit against BP and Transocean, accusing BP specifically of negligence.

"The responsibility for safety on the drilling rig is with Transocean," Hayward added. "It is their rig, their equipment, their people, their systems, their safety processes."

He insisted that, despite reports to the contrary, BP has not resisted attempts at tightening safety regulations.

"We welcome tighter safety regulations. But we'd like them to be applied in a way that makes them practically impermeable."

Before the explosion, Hayward had announced a "significant" discovery of at least 50 million barrels of oil.

"Of course, all of that is completely irrelevant in the context of what we're now dealing with," he said.

A controlled attempt to burn off part of the spill was successful Wednesday, Landry said. The burnoff is part of the effort to prevent the spread of oil from an underwater well that was broken open when the drill rig Deepwater Horizon blew up and sank last week.

BP and the Coast Guard corralled part of the oil slick using a 500-foot, specially designed boom, and then set it ablaze. The flames were expected to destroy between 50 percent to 90 percent of the oil in that section, and winds were expected to blow the resulting cloud of smoke and soot out to sea, Lt. Cmdr. Matt Moorlag, a Coast Guard spokesman, said before the burn.

"It's a historically proven technique, and it has multiple preventative safety measures in place to ensure that that burn area remains controlled," Moorlag said.

The slick stretched about 100 miles across the north-central gulf Wednesday afternoon and had advanced to within 16 miles of the mouth of the Mississippi River, the U.S. Coast Guard reported.

The oil spill has the potential to become one of the worst in U.S. history, Landry said earlier.

Louisiana Gov. Bobby Jindal said the slick, which is about 30 miles wide at some points, was expected to hit the state's southeastern shoreline later this week.

Jindal said the state has asked for 55,000 feet of booms to keep oil away from the marshy, environmentally delicate coast that's rich in shellfish and wildlife.

"We want to approach this situation the same way we would approach a hurricane or other natural disaster," he said. "We think it's best to hope for the best, but prepare for the worst."

The slick left many who draw their living from the water and coastal wetlands "watching and praying," said Tony Fernandez, owner of the Breton Sound Marina near Hopedale, Louisiana.

"For the most part, what we're doing is mostly waiting," Fernandez said. "There's not much that laypeople can do with this."

Most of the slick is a thin sheen on the water's surface. About 3 percent of it is a heavy, pudding-like crude oil.

Efforts already are under way to position boom material around sensitive ecological areas. Five staging areas have been set up on land, stretching from Venice, Louisiana, to Pensacola, Florida.

"If it reaches the shoreline, ourselves and the Coast Guard ... will deal with it," BP's Hayward said. "And we will clean it up, if we get to that position."