NEW ORLEANS - A containment cap was capturing more and more of the crude pouring from a ruptured oil well in the Gulf of Mexico, but despite that encouraging development, the federal government's point man warned Sunday that the crisis could stretch into the fall.

The inverted funnellike cap is being closely watched for whether it can make a serious dent in the flow of new oil. Coast Guard Adm. Thad Allen, the national incident commander, reserved judgment, saying he did not want to risk offering false cheer.

Instead, he warned on CBS's Face the Nation that the battle to stop the spill was 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. Until then, oil will continue to spew.

"But even after that, there will be oil out there for months to come," Allen said.

"This will be well into the fall," he said. "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."

Since it was placed over the busted well Thursday, the cap has been siphoning an increasing amount of oil. On Saturday, it funneled 441,000 gallons to a tanker on the surface, up from about 250,000 gallons it captured Friday.

But it is not clear how much is still escaping from the well that federal authorities at one point estimated was leaking from 500,000 to one million gallons a day.

Since the spill began nearly seven weeks ago, 23 million to 49 million gallons of oil have leaked into the gulf by government estimates.

The prospect that the crisis could stretch beyond summer was devastating to residents along the gulf, who are seeing thicker globs of oil show up in increasing volume all along the coastline.

At Ruth Dailey's condominium in Gulf Shores, Ala., floors already are smeared with dark blotches of oil.

"This is just the beginning," she said. "I have a beachfront condo for a reason. With this, no one will want to come."

Kelcey Forrestier, 23, of New Orleans, said she no longer trusted the word of either BP P.L.C. or the U.S. government in laying out the extent of the spill. But it was 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 chief executive officer 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, which 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.

On Sunday, BP said it had closed one of four vents that are allowing oil to escape and preventing the water intake. The company said some of the remaining vents might remain open to keep the cap system stable.

Hayward told the BBC that the company hoped a second containment system would be in place by next weekend. Allen told CBS that the oil would stop flowing only when the existing well was plugged with cement once the relief wells had been completed.

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

The plan would use lines and pipes that once injected mud down into the well - one of several failed efforts over the last six-plus weeks to contain the leak - 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 300 feet below the ocean's surface - far enough below the water 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 are simply designed to capture what is leaking until the relief wells can be drilled. The spill began when the Deepwater Horizon rig exploded 50 miles off the coast of Louisiana on April 20, killing 11 workers.

In the last week, increasing quantities of sludge have been making their way farther east, washing up on some of the region's hallmark white-sand beaches and coating marshes. An observation flight spotted a sheen of oil 150 miles west of Tampa, Fla., but officials said Sunday they did not expect it to reach western Florida any time soon.

The sight and smell of oil along the shoreline undermined any consolation offered by reports Sunday of progress at the wellhead. Gulf residents voiced frustration with the apparent holes in cleanup efforts.

At Gulf Shores, Dailey walked along a line of oil mixed with seaweed that stretched as far as the eye could see. Clumps of seaweed hiding tar balls make the scene appear better than it really is, she said.

"They're lying when they say they're cleaning these beaches," said Dailey, of Huntsville. "They're saying that because they still want people to come."

Eventually, workers used a big sand-sifting machine to clean the public beach, leaving it spotless, at least for a while.

At Pensacola Beach, Fla., the turquoise waves were flecked with floating balls of tar. Buck Langston, who has been coming to the beach to collect shells for 38 years, watched as his family used improvised chopsticks to collect the tar in plastic containers.

"Yesterday it wasn't like this, this heavy," said Langston, of Baton Rouge, La. "I don't know why cleanup crews aren't out here."

Allen urged understanding: "Everyone wants certainty," he said. "With an oil spill like this, there isn't any."