Containment operations at the blown-out oil well in the Gulf of Mexico are now capturing about 2.3 million litres of oil daily, and that could double by next week, says U.S. Coast Guard Admiral Thad Allen.

National incident commander Admiral Thad Allen speaks during a news conference at the Homeland Security Department in Washington on Wednesday. (Pablo Martinez Monsivais/Associated Press)

Speaking to reporters in Washington, Allen said BP is bringing in a second oil processing vessel that will boost capacity. A North Sea shuttle tanker will also arrive within days to assist in the transport of the oil.

"We continue to make progress," he said.

Officials still don't have a firm handle on how much oil is actually escaping from the ground.

"I'm not going to declare victory on anything until I have absolute numbers," said Allen.

He also said Wednesday that he has written to BP to call for more detail and openness in the company's handling of damages claims.

"This is not a competency that comes with large oil companies," Allen told reporters. "We feel it’s our responsibility from the oversight role we have with BP to make sure this is done effectively."

Underwater plumes disputed

Earlier in the day , a BP executive disputed claims of large underwater oil plumes escaping into the Gulf of Mexico from the blown-out well.

Gas and oil continue to surge from the containment cap at the Deepwater Horizon oil spill site in the Gulf of Mexico in this image captured from a BP live video feed on Wednesday. (BP/Reuters) Speaking on NBC's Today show, BP chief operating officer Doug Suttles insisted the oil concentrations were "very low."

Suttles said it "may be down to how you define what a plume is here."

The statement came a day after the U.S. government said water tests showed the existence of underwater plumes.

On Tuesday, National Oceanic and Atmospheric Administration administrator Jane Lubchenco said tests performed by a University of South Florida research vessel showed the presence of oil roughly 1,000 metres under the surface of the water and about 68 kilometres northeast of the spill site.

Tens of millions of litres of oil have spilled into the Gulf since the Deepwater Horizon rig exploded 80 kilometres off the coast of Louisiana 51 days ago. There is no consensus on exactly how much oil is streaming out of the damaged well, but federal officials have estimated the flow could be between 12,000 and 25,000 barrels a day.

Political response

Meanwhile, attention will also be on Washington, where four Democratic-controlled committees will hold meetings Wednesday on spill cleanup, financial responsibility, liability caps and offshore drilling safety.

Nancy Pelosi, the Speaker of the House of Representatives, has given committee heads until July 4 to draw up new legislation to grapple with the spill and environmental disasters.

The move is part of the Democrats' political response amid growing public frustration over the government's handling of the Gulf spill.

A Washington Post poll found that the percentage of respondents who say President Barack Obama doesn't understand the problems of people like them has hit 48 per cent — the highest level since he took office in January 2009.

Obama is slated to visit the Gulf region next week, for his fourth trip to the region since the spill started.

With files from The Associated Press