Under promising with hopes of over delivering, BP said Sunday that it is making progress on what could prove its most effective effort yet to contain the Gulf oil leak, but cautioned that the verdict could be several days away.

A new cap being placed atop the gusher is intended to provide a tight seal and might eventually allow the oil giant to capture all the crude leaking from the well for the first time since an April 20 oil rig explosion set off the environmental crisis. But several prior failed attempts to stop the leak have made BP PLC careful to keep expectations grounded.

"We're pleased with our progress," said BP senior vice president Kent Wells, who then hastened to add the operation was still expected to last up to six more days.

Asked during a conference call if the new cap and collection efforts would end the spilling of oil into the Gulf, Wells said only that BP will capture all the oil "at some point."

Wells said BP may have to bring another vessel back online and add additional collection capacity in order to stop the oil flow altogether.

Officials won't be satisfied the cap is working until they've run tests on whether it can withstand the tremendous pressure of oil pushing up from below the seafloor, Wells said.

"We've tried to work out as many of the bugs as we can. The challenge will come with something unexpected," he said.

The well has been gushing largely unchecked since an old, leaky cap was removed from the wellhead Saturday to make way for the new one. Between 88 million and 174 million gallons have already spilled into the Gulf, according to federal estimates.

Wary Gulf residents reserved judgment about BP's latest effort and said the damage already done to the environment, fishing and tourism will haunt the region for a long time either way.

"At this point, there have been so many ups and downs, disappointments, that everybody down here is like, 'We'll believe it when we see it,'" said Keith Kennedy, a charter boat captain in Venice, La.

Robotic submarines finished removing a busted piece of pipe that was bolted around the leak around 3 am Sunday. That paved the way for the installation of a pipe-like connector called a flange spool that will sit on top of the spewing well bore. The new cap would be mounted on top of that connector and have flexible pipes leading up to surface ships.

The work was being closely monitored at the White House, where President Barack Obama is being briefed multiple times a day, adviser David Axelrod said on ABC's "This Week."

"We have every reason to believe that this will work," he said.

The new cap will be aided in containing the leak by the arrival of the Helix Producer, a vessel that will be able to take in about 1 million gallons of crude per day by Tuesday after gradually ramping up. The Helix connected to flexible pipes from the well Friday, and crews have been running tests since then.

Like another vessel already operating, the Q4000, the Helix will take in oil through connections beneath the new seal. Once the new cap is affixed, two other vessels are to connect to it for their oil collection.

Ultimately, the four vessels collecting oil from the leak would have a rough capacity of about 2.5 million to 3.4 million gallons a day — enough to capture all the oil leaking, if federal estimates are right. Getting all the vessels on the task will take about two to three weeks.

The hurricane season that lasts through November could interfere. There are no storms forecast now, but if one blows through, the ships collecting the oil may have to leave and crude would spew again for days into the water.

If the new cap is a complete success in stopping the leak, that will be a first.

In May, BP tried to drop a four-story, 100-ton steel-and-concrete box on the leak, but abandoned it when it was encased by ice-like crystals. That was followed by a mile-long siphon tube designed to suck up oil escaping the leak. That, in turn, was scrapped after it managed to suck up only about 900,000 gallons after roughly a week of operation.

The most anticipated effort was the so-called "top kill," in which mud and cement were pumped down from above the leak. After about three days of the strategy, BP announced on May 29 it had failed.

Then came the cap that was removed Saturday. That device didn't form a perfect seal on the jagged pipe that was cut to make room for it, and the inexact fit meant that it was able to collect only 1 million gallons or so of oil a day.

The new, tighter cap is not intended to be the permanent fix to the problem.

Relief wells are being dug for the permanent fix, a "bottom kill" in which heavy drilling mud and cement are pumped in from below the broken wellhead.

The effort to drill relief wells was moving ahead of schedule, Wells said Sunday. BP and government officials have said the wells are expected to be completed sometime around mid-August.

The new cap, or "Top Hat 10," weighs some 150,000 pounds (68,000 kilograms) . It is designed to fully seal the leak and provide connections for new vessels on the surface to collect oil. The cap has valves that can restrict the flow of oil and shut it in, if it can withstand the enormous pressure.

Former Sen. Bob Graham of Florida, co-chair of the national oil spill commission, said capping the spill would be a relief.

"But the problem is still going to be there to deal with the enormous amount of petroleum put in the Gulf of Mexico," he said during a stop at a local emergency operations center in the Florida Panhandle town of Southport.

Crews are working to skim oil from the Gulf's surface and remove it from the coast, but the job is enormous. People on shore who depend on the Gulf for their livelihood are paying attention to the latest effort but wonder if the damage already done by the spill is too much.