BP has succeeded in capturing "some" oil and gas by inserting a mile-long tube into the main Gulf of Mexico leak, but would not say if it was a significant percentage of the gusher or just a dribble.

Despite the uncertainty, it was still the first tangible sign of success in more than three weeks of efforts to prevent at least 210,000 gallons of oil from spewing unabated into the sea each day and feeding a massive slick off the coast of Louisiana.

BP senior executive vice president Kent Wells refused to be drawn on quantity Sunday, but confirmed that after a temporary hitch in which the tube became dislodged overnight, siphoning operations were up and running once again.

"We will look to... capture as much of the oil as we can," he told reporters in Houston, Texas. "At this point, we don't know what percentage we will capture" by the process, in which the oil was sucked up as if through a straw to the giant ship.

A BP statement said simply that the four-inch (10-centimeter) diameter tube inserted into the 21-inch leaking pipe using undersea robots had captured "some amounts of oil and gas."

Wells added that the BP crews "don't have any idea at this point" how much crude is being collected and would only have a better estimate in coming days.

"The oil was stored on board the Discoverer Enterprise drill ship 5,000 feet (1,500 meters) above on the water's surface, and natural gas was burned through a flare system on board the ship," the statement said.

The Barack Obama administration seemed unimpressed however, saying BP's latest efforts, even if they manage to slow the leak, would not permanently stop the underwater geyser.

"This technique is not a solution to the problem, and it is not yet clear how successful it may be," said Homeland Security Secretary Janet Napolitano and Interior Secretary Ken Salazar said in a statement.

"We are closely monitoring BP's test with the hope that it will contain some of the oil, but at the same time, federal scientists are continuing to provide oversight and expertise to BP as they move forward with other strategies to contain the spill and stop the flow of oil," they said.

The Deepwater Horizon rig, leased by BP from Transocean, has been gushing oil since an explosion on April 20 ripped through the drilling platform and caused it to sink two days later. Eleven workers were killed.

Fresh analysis of enormous plumes of oil under the surface suggest the spill may be far worse than previously estimated.

One was reported to be 10 miles (16 kilometers) long, three miles wide and 300 feet thick.

Researchers from the National Institute for Undersea Science and Technology said the plumes were "perhaps due to the deep injection of dispersants which BP has stated that they are conducting."

Response crews have so far used some 560,000 gallons of the controversial chemical dispersants, spraying them onto surface oil and also directly into the leak in a bid to break up the oil.

"The oil still exists, it's just spread in smaller pieces," Aaron Viles, campaign director for the Gulf Restoration Network, a coalition of environmental groups, told AFP. "It could have a significant impact on the marine life of the Gulf of Mexico."

University of Georgia researcher Samantha Joye, who is on a scientific mission to gather details about the looming environmental disaster, told The New York Times that oxygen levels have dropped 30 percent near the plumes, in an "alarming" trend that is endangering marine life.

But Andrew Gowers, head of group media for BP, dismissed reports that "speculate" on the giant plumes. He said officials "had no confirmation" of oil clumping together in mid-ocean areas.

On Sunday a large concert in New Orleans was drawing crowds to support Gulf fishermen, whose livelihoods are threatened by the oil spill, with rocker Lenny Kravitz heading the line-up.

Officials said some 19,000 personnel and more than 650 vessels have been deployed to try to mitigate the negative effects of the spill on the Gulf shoreline and wildlife.

Meanwhile, the US Coast Guard told AFP that oil was washing ashore in at least two new locations -- Whiskey Island, Louisiana and Long Beach, Mississippi.

Engineers are mulling several different options to seal the main leak, which has spewed out an estimated five million gallons so far according to the most conservative estimates, and prevent the giant slick from destroying ecologically fragile wetlands and nature reserves.

A relief well that would divert the flow and allow the well to be permanently sealed may not be ready until August.