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Crude oil, seen here in the Gulf of Mexico, is fouling beaches as far east as the Florida Panhandle. ((Naval Research Laboratory/U.S. Geological Survey))

A containment cap is collecting about one-third of the oil gushing daily from BP's damaged deepwater well in the Gulf of Mexico, the U.S. Coast Guard said Saturday.

Admiral Thad Allen said 6,000 barrels of oil (954,000 litres) were captured in a 24-hour period after the procedure began Friday, adding that engineers want to gradually raise that amount in the next few days by closing vents.

Oil is being allowed to escape from four vents to stop water from entering and forming hydrates that interfere with the procedure.

According to government estimates, the BP well has leaked 189.3 million litres of oil in the last six weeks.

NOAA monitors underwater oil plumes

Despite limited success with the containment cap, at least two massive oil plumes are fouling water under the surface, threatening deep-sea coral communities.

Research vessels will create a model to map the extent of the plumes, Allen told reporters at a news conference in Theodore, Ala.

Vessels from the National Oceanic and Atmospheric Administration have been scouting an area near the oil rig that sank April 22, and more ships will return to test the water at different depths, he said.

"They’ll put together a profile, based on all the data, so we can understand what the picture looks like for the Gulf area in relation to what kind of hydrocarbons are present in the water column and come up with a general model of what’s going on in the Gulf," Allen said.

Tests conducted by scientists at the University of South Florida show one of the plumes is nearing a large underwater canyon whose currents fuel the food chain in Gulf waters off Florida.

On Friday, Ernst Peebles, a USF biological oceanographer and chief scientist aboard the Weatherbird II research vessel, said samples were drawn from two layers of oil. One measured 30 metres thick at a depth of about 400 metres.

It's believed the other layer is thicker, at a depth of 975 metres.

Obama promises cleanup effort 'on every front'

In his weekly radio and internet address, President Barack Obama said his administration has put in place the largest response to an environmental disaster in U.S. history.

He said the government had been "mobilized on every front," with more than 1,900 ships and 20,000 people helping clean up the spill.

Oil from the leak has reached the ecologically sensitive shores of Louisiana, Mississippi, Alabama and Florida. Tar balls continued to wash ashore on the white-sand beaches of Pensacola, Fla., for a second day on Saturday.

"We want to disperse oil at the source," Allen said. "We're trying to limit, if at all possible, any dispersant application on the surface because we've used so much up there, more than was ever contemplated with a spill this size."

Officials have released just over a four million litres of chemicals in the water to disperse the oil.

"We've reached the million-gallon threshold on dispersants," Allen said.

"There's some public concern over the implication of dispersants," he said. "It's preferable to use a dispersant rather than have the oil because the toxicity is much less, but we are mindful that there are some toxic impacts of the dispersant.

"We're trying to focus it now on the subsea area where the oil is actually coming out of the wellhead."

A federal panel of about 50 experts is recommending the continued use of chemical dispersants to break up the oil, despite its harm to plankton, larvae and fish.

Panel member Ron Tjeerdema said Friday they decided the animals harmed by the chemicals underwater had a better chance of rebounding quickly than birds and mammals on the shoreline. Tjeerdema chairs the Department of Environmental Toxicology at the University of California, Davis.

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With files from The Associated Press