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The cap over a blown-out oil well is capturing more and more of the crude pouring into the Gulf of Mexico, but that bit of hope was tempered Sunday by a sharp dose of pragmatism as the federal government's point man warned the crisis could stretch into the fall.

The inverted funnel-like cap is being closely watched for whether it can make a serious dent in the flow of new oil. Coast Guard Admiral Thad Allen, overseeing the government's response to the spill, reserved judgment, saying he didn't want to risk offering false encouragement.

Instead, he warned on CBS's Face the Nation that the battle to contain the oil is likely to stretch into the fall. The cap will trap only so much of the oil, and relief wells being drilled won't be completed until August.

In the meantime, oil will continue to spew out. "But even after that, there will be oil out there for months to come," Allen said.

Millions of litres of leaked oil in Gulf 

"This will be well into the fall. This is a siege across the entire Gulf. This spill is holding everybody hostage, not only economically but physically. And it has to be attacked on all fronts," he said.


Cleanup crews walk past beachgoers as they look for globs of oil on Dolphin Island, Alabama. ((Sean Gardner/Reuters))

Since it was placed over the broken well on Thursday, the cap has been diverting an increasing amount of oil. On Saturday, it funnelled about 1.7 million litres to a tanker on the surface, up from nearly a million litres it captured Friday.

But it's not clear how much is still escaping from the well that federal authorities at one point estimated was leaking between 1.9 million and 3.8 million litres a day. Since the spill began nearly seven weeks ago, roughly 87 million to 185 million litres of oil have leaked into the Gulf.

Once the cap is fully operational, if it is ultimately successful, it could capture a maximum of about 2.4 million litres of oil a day.

Kelcey Forrestier, 23, of New Orleans, said she no longer trusts the word of either BP or the U.S. government in laying out the extent of the spill. But it is clear to Forrestier, just coming in off the water at Okaloosa Island, Fla., that the spill and its damage will last long into the future.

"Oil just doesn't go away. Oil doesn't disappear," said Forrestier, who just earned a biology degree. "It has to go somewhere and it's going to come to the Gulf beaches."

BP has plans for 2nd containment system

BP chief executive Tony Hayward told the BBC on Sunday that he believed the cap was likely to capture "the majority, probably the vast majority" of the oil gushing from the well. The gradual increase in the amount being captured is deliberate, in an effort to prevent water from getting inside and forming a frozen slush that foiled a previous containment attempt.

Allen was reluctant to characterize the degree of progress, saying much more had to be done.

"We need to underpromise and overdeliver," he said.

BP trying to set up second containment system

BP engineers must next try to close vents on the containment cap that are allowing oil to escape and preventing that water intake. Hayward told the BBC that the company hopes a second containment system will be in place by next weekend.

Allen told CBS that the oil would stop flowing only when the existing well is plugged with cement once the relief wells have been completed.

Besides installing the containment cap, BP officials have said they want a second option for diverting oil by next weekend.

The plan would use lines and pipes that previously injected mud into the well — one of several failed efforts over the past six-plus weeks to contain the leak — and instead use them to suck up oil and send it to a drilling rig on the ocean surface.

BP also wants to install by late June another system to help cope with hurricanes that could roar over the site of the damaged well. When finished, there would be a riser floating about 100 metres below the ocean's surface, far enough down so it would not be disturbed by powerful hurricane winds and waves but close enough so ships forced to evacuate could easily reconnect to the pipes once the storm has passed.

None of these fixes will stop the well from leaking; they're simply designed to capture what's leaking until the relief wells can be drilled.