New Orleans ? Underpromising with hopes of overdelivering, 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 on line and add additional collection capacity 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.

Robotic submarines finished removing a busted piece of pipe that was bolted around the leak around 3 a.m. 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.