HOUSTON/VENICE, Louisiana (Reuters) – Anger, scepticism and accusations of lying washed over energy giant BP Plc on Friday as it desperately pursued efforts to contain a month-old seabed well leak billowing crude oil into the Gulf of Mexico.

U.S. lawmakers and scientists have accused BP of trying to conceal what many believe is already the worst U.S. oil spill, eclipsing the 1989 Exxon Valdez accident in Alaska. It represents a potentially environmental and economic catastrophe for the U.S. Gulf coast.

The London-based energy giant, facing growing federal government and public frustration and allegations of a coverup, said its engineers were working with U.S. government scientists to determine the real size of the leak, even as they fought to control the still-gushing spill with uncertain solutions.

President Barack Obama’s administration was keeping up the pressure on BP to do everything possible to stop the leak.

“We are facing a disaster, the magnitude of which we likely have never seen before, in terms of a blowout in the Gulf of Mexico,” White House spokesman Robert Gibbs told reporters. “And we’re doing everything humanly possible and technologically possible to deal with that.”

BP’s next planned step is a “top kill” — pumping heavy fluids and then cement into the gushing well to plug it. That operation could start next week, perhaps on Tuesday, BP Chief Operating Officer Doug Suttles said.

Adding to the confusion, BP revised downward on Friday an estimate from Thursday that one of its containment solutions — a 1 mile (1.6 km)-long siphon tube inserted into the larger of two seabed leaks — was capturing 5,000 barrels (210,000 gallons/795,000 litres) of oil per day.

A BP spokesman said the amount of crude oil it sucked from the leak fell to 2,200 barrels (92,400 gallons/350,000 litres) a day in the 24-hour period ended at midnight on Thursday.

“The rate fluctuates quite widely on this tool,” Suttles told reporters at a briefing in Robert, Louisiana.

Many scientists dismiss an original 5,000 bpd estimate of the total leaking oil — often defended by BP executives — as ridiculously low and say it could be as high as 70,000 barrels (2.9 million gallons/11 million litres) per day or more.

“There’s a huge amount of uncertainty around that number and it could have a fairly wide range,” Suttles said. A federal panel will release its estimate of the actual flow rate as early as next week, a Coast Guard official said.

“HOT POTATO”

“It’s very clear that BP has not been telling the truth,” Massachusetts Democratic Representative Ed Markey told CNN.

BP denied any coverup and said some third-party estimates of the leak were inaccurate. The company’s shares fell more than 4 percent in London.

Michael Gordon, chief executive of Gordon Strategic Communications, a corporate and crisis public relations firm in New York, called BP’s handling of the spill “a case study in failed crisis communications.”

“It would not have been possible for them to handle this worse. They are not taking sufficient responsibility for what happened. They’ve played a game of ‘hot potato’ with the other companies involved,” he said, referring to BP’s public trading of blame with its partners in the drilling of the well.

A month after the well blowout and rig explosion that unleashed the catastrophic spill, sheets of rust-coloured heavy oil are starting to clog fragile marshlands on the fringes of the Mississippi Delta, damaging fishing grounds and wildlife.

Scientists fear parts of the huge fragmented surface slick will be sucked to the Florida Keys and Cuba by ocean currents.

At a briefing in Mobile, Alabama, Coast Guard Incident Commander Captain Steven Poulin said an overflight on Thursday of the oil slick showed that while “minor portions of sheen” were in the Loop Current there was “really no trail or elephant trunk” of oil extending down into the moving current flow.

Markey said it was clear BP was only siphoning off “just a small fraction.”

“BP has mismanaged this entire incident from day one,” he said. “They should not be trusted.”

BP spokesmen say the original leak estimate came from the U.S. National Oceanic and Atmospheric Administration, one of its federal partners in the joint spill response.

“I understand the frustration,” Suttles told CBS. “We’re supplying information.”

LIABILITY ISSUE

“It’s obvious they are trying to limit information to protect their economic liability,” said Markey, chair of the Select Committee on Energy Independence and Global Warming.

In a sign of the Obama administration’s mounting anger and frustration, senior U.S. official have demanded BP share more data on the spill with them, accusing the company of falling short in keeping the government and public informed.

There were signs of growing tension between BP officials and the U.S. government at the briefing in Robert. As Suttles described the “massive amount of information being gathered through this exercise,” Coast Guard Rear Admiral Mary Landry interrupted.

“I have to challenge the word ‘exercise,’ Doug,” Landry said. “This is a full response.”

BP said it was working with a newly created Flow Rate Technical Team to determine the exact amount of oil escaping.

Suttles said BP had spent almost $700 million on the spill response and had “thrown absolutely everything” at the job. BP also is drilling a relief well to try to plug the leak but it probably would not be finished until August.

On the Louisiana coast, fishermen counted the cost to their livelihoods. “This is going to keep killing stuff and it will make whole areas incapable of supporting marine life,” said George Barisich, president of the United Commercial Fishermen’s Association.

BP has promised to pay legitimate damages claims and faces billions of dollars in expected cleanup and damages costs.

(Additional reporting by Anna Driver and Chris Baltimore in Houston, Matt Spetalnick in Washington; Tom Bergin in London; Writing by Pascal Fletcher; Editing by Bill Trott)