(Corrects to show BP shares closing up just over 1 percent) By Anna Driver and Matthew Bigg
HOUSTON/VENICE, Louisiana (Reuters) - BP <:BP.LO:>Plc said on Thursday it was siphoning off more of the oil gushing from its ruptured Gulf of Mexico well, but the energy giant faced "cover-up" allegations over its struggling response to the catastrophic month-old spill.
"The oil plume escaping from the riser pipe has visibly declined today," BP spokesman Mark Proegler said after the company announced that a mile-long tube tapping into the larger of two leaks from the well was now capturing 5,000 barrels (210,000 gallons/795,000 litres) per day of oil.
However, a live video feed of the leak, provided by BP, showed a black plume of crude oil still billowing out into the deep waters.
"It's just not working," U.S. Senator Barbara Boxer, who heads the Environment and Public Works Committee, told CNN as she watched the BP video. The California Democrat denounced a "cover-up" of the real size of the oil spill.
The increased amount of oil BP reports being captured tallied with an estimate -- originally given by government and BP officials overseeing the spill response -- for the total crude leaking from the Macondo well that blew out on April 20. The resulting rig explosion killed 11 workers.
Proegler and other BP spokesmen made clear the increased containment, while an advance, was not siphoning all the escaping oil. "We're not claiming that we stopped it -- although that is our final objective. We're saying that this is what we're capturing now," he said.
DISPUTE OVER LEAK DATA
The U.S. government, grappling with a potentially huge environmental and economic disaster, said on Thursday it would not rely only on data given by well owner BP, but would make its own checks on the total size of the leak.
Boxer and Florida Democrat Senator Bill Nelson cited estimates by scientists who believe the real size of the leak is much higher -- as much as 70,000 barrels (2.9 million gallons/11 million litres), per day or even more.
"The truth needs to be told ... At some point we need to stop all this cover-up," Boxer said.
With heavy oil sloshing ashore in Louisiana's fragile marshlands, heralding an ecological catastrophe, President Barack Obama's administration faces criticism that it has been too willing to accept BP's estimates of the gushing oil.
Popular anger against BP has risen, especially among Gulf Coast residents -- shrimpers, fishermen and tourism operators -- who fear their livelihoods will be devastated by the spill.
BP's shares, after initially falling in London trading, closed just over 1 percent up.
BP CEO Tony Hayward has been quoted recently by British media as playing down the size of the spill and its environmental impact.
"We're not depending on what BP is telling us," Interior Secretary Ken Salazar told CNN.
Steve Wereley, associate mechanical engineering professor at Purdue University, said most independent estimates of the spill flow were "considerably higher than BP's."
"This is not rocket science," Wereley told a U.S. congressional panel on Wednesday.
BP spokesmen said the original 5,000 bpd estimate was given by the National Oceanic and Atmospheric Administration.
Salazar said BP was responsible for damages so getting accurate data was essential. "It's a grave and a very serious situation and we're taking nothing for granted," Salazar told NBC's "Today" show.
Wildlife and environmental groups have accused BP of holding back information on the real size and impact of the growing slick and on the quantity and toxicity of dispersants being used against the oil, both above and below the water.
A BP spokesman said the company had received a letter from the U.S. Environmental Protection Agency asking it to switch to other dispersants. "We are looking at what we can to do to comply," he said.
The spokesman was responding to a Washington Post report that said the EPA informed BP officials late on Wednesday that the company had 24 hours to choose a less toxic form of chemical dispersants to break up the oil spill.
The report, citing government sources, said U.S. officials worried about the environmental impact of the dispersants.
Sheets of heavy oil came ashore in Louisiana's wetlands on Wednesday for the first time since the rig exploded a month ago. The marshes are nurseries for shrimp, oysters, crabs and fish that make Louisiana the top commercial seafood producer in the continental United States. Fishing is banned in a large part of the Gulf waters because of the spill.
In Pass-a-Loutre, La., thick sheets of gooey brown oil swamped islands of marsh grass at the southern tip of a Mississippi River channel on Thursday.
"To see the extent to which it is oiled and and the depth into the island is stunning," said Maura Wood of the National Wildlife Federation's Coastal Louisiana Restoration Project.
The oil pollution covers only a fragment of the vast network of waterways, channels and islands that make up the Delta region, but environmentalists fear it is just the start.
"It's going to take a long time for us to recover from BP's mess," boat captain Richard Blink said. (Additional reporting by Jeff Mason, Tabassum Zakaria, Vicki Allen, Tom Bergin, Tom Brown and Pascal Fletcher; Writing by Jane Sutton and Pascal Fletcher; Editing by Doina Chiacu