NEW ORLEANS - Oil was spewing largely unchecked into the Gulf of Mexico as BP crews claimed progress Sunday in the first stages of replacing a leaky cap with a new containment system they hope will finally catch all the crude from the busted well.

There's no guarantee of success for such a delicate operation nearly a mile (1.6 kilometres) below the water's surface, officials said, and the permanent fix of plugging the well from the bottom remains slated for mid-August.

"It's not just going to be, you put the cap on, it's done. It's not like putting a cap on a tube of toothpaste," Coast Guard spokesman Capt. James McPherson said.

Robotic submarines on Saturday removed the cap that had been placed on top of the leak in early June to collect the oil and send it to surface ships for collection or burning. BP aims to have the new, tighter cap in place as early as Monday and said that, as of Sunday morning, the work was going according to plan. BP hopes the capping operation will be done within three to six days.

Kent Wells, a BP senior vice-president, said during a Sunday morning news briefing he was pleased with the progress but cautioned that unforeseen bumps could lie ahead.

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

If tests show the new cap can withstand the pressure of the oil and is working, the Gulf region could get its most significant piece of good news since the April 20 explosion on the BP-leased Deepwater Horizon rig, which killed 11 workers. Since then, between 88 million gallons (333 million litres) and 174 million gallons (659 million litres) of oil have spilled into the Gulf, according to federal estimates.

It would be only a temporary solution to the catastrophe. Hope for permanently plugging the leak lies with two relief wells, the first of which should be finished by mid-August.

And 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.

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

"We have every reason to believe that this will work," he said. At the same time, Axelrod acknowledged that BP's engineers are in "uncharted waters" when it comes to dealing with the leak.

With the cap removed Saturday at 12:37 p.m. CDT (1737 GMT), oil flowed freely into the water, collected only by the Q4000 surface vessel, with a capacity of about 378,000 gallons (1.43 million litres). That vessel should be joined Sunday by the Helix Producer, which has more than double the Q4000's capacity.

But the lag could be long enough for as much as 5 million gallons (19 million litres) to gush into already fouled waters. Officials said 46 large skimmers had collected about 1 million gallons (3.8 million litres) of oily water from the surface above the well site as of Sunday morning.

The process begun Saturday has two major phases: removing equipment currently on top of the leak and installing new gear designed to fully contain the flow of oil.

BP on Sunday said it had successfully removed the top flange that had only partially completed the seal with the old cap, almost a day earlier than a previous estimate.

Now that the top flange is removed, BP is considering whether it needs to bind together two sections of drill pipe that are in the gushing well head. The step following that involves lowering a 12-foot (3.6-meter) -long piece of equipment called a flange transition spool onto the well head and bolting it to the bottom flange still in place.

After the spool is bolted in place, the new cap — called a capping stack or "Top Hat 10" — can be mounted. The equipment, weighing some 150,000 pounds (68.000 kilograms), 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.

That will be one of the key items for officials to monitor, said Paul Bommer, a professor of petroleum engineering at the University of Texas at Austin.

"If the new cap does work and they shut the well in, it is possible that part of the well could rupture if the pressure inside builds to an unacceptable value," Bommer wrote in an email Saturday.

Ultimately, BP wants to have four vessels collecting oil within two or three weeks of the new cap's installation. If the new cap doesn't work, BP is ready to place a backup similar to the old one on top of the leak.

The company originally planned to bring the Helix Producer on site and install the new cap at different times, but combined the two following forecasts of calm weather for about seven to 10 days.

The new vessels will all be connected to the gusher through flexible hoses that will allow them to disconnect and sail away much quicker in the event of a hurricane. Prior to the new lineup at the site, officials estimated they would need five days to remove everything in advance of a major storm; the new setup should cut that to two.

The government estimates 1.5 million gallons (5.7 million litres) to 2.5 million gallons (9.5 million litres) of oil a day are spewing from the well, and the previous cap collected about 1 million gallons (3.8 million litres) of that. With the new cap and the new containment vessel, the system will be capable of capturing 2.5 million gallons (9.5 million litres) to 3.4 million gallons (12.9 million litres) — essentially all the leaking oil, officials said.


Associated Press Writers Vicki Smith in New Orleans and Carrie Schumaker in Washington contributed to this report.