Enlarge By Evan Vucci, Associated Press President Obama, LaFourche Parish president Charlotte Randolph, right, and U.S. Coast Guard Admiral Thad Allen, National Incident Commander for the BP Deepwater Horizon oil spill, look Friday at booms laid out to collect oil during a tour of areas impacted by the Gulf Coast oil spill.  HOW IT WORKS

Obama gave them his pledge that the federal government will keep helping until the disaster is ended.

During a visit Friday to Louisiana, he toured a beach where tar balls are washing ashore and attended a briefing at a Coast Guard station in Grand Isle, a small barrier island town south of New Orleans.

Obama said that Coast Guard Admiral Thad Allen, who is leading the federal response, will get all the help he needs.

Friday's trip was the president's second to the coast since the BP-leased oil rig Deepwater Horizon exploded April 20, killing 11 workers and triggering the spill.

Residents angry and frustrated over the slow pace of efforts to stop the leak did not expect the visit to change much.

"It's a dog and pony show. What can he really do?" said Billy Ward, 53, who goes to Grand Isle with his family every weekend to stay in their beach house. "If he wants to do something, let him get out there and pump some mud and cement into that hole. Just fix it. Help us."

Obama arrived 38 days into the disaster, as BP worked to plug the leak with drilling fluid, dubbed mud. That procedure is expected to continue for a couple of days before its outcome is clear.

Virginia Smith, 36, wasn't impressed.

"I like the man, but I personally feel he's only here to please everybody," she said. "He's not here to make any changes."

In Washington, Secretary of State Hillary Rodham Clinton said the government was still evaluating offers from 17 countries and organizations for such things as technical expertise and equipment. The Coast Guard hasn't yet accepted any of the foreign help, but BP has accepted booms and skimmers from Mexico and Norway.

CEO Tony Hayward said on the CBS "Early Show" that his confidence level in the well-plugging bid remains at about 60 to 70%.

Workers resumed pumping heavy mud into the gushing oil well in the Gulf of Mexico after temporarily suspending the effort to assess the situation, oil company BP said late Thursday.

The latest attempt to block the well came as the government confirmed that the undersea gusher of toxic oil and gas is far bigger than what had been initially estimated, making the offshore environmental disaster the largest in the nation's history.

The U.S. Geological Survey said three teams of scientists concluded that between 504,000 and 798,000 gallons a day had been billowing out of the mud a mile beneath the sea. Initial estimates were 210,000 gallons spilling per day. That means 18 million to 30 million gallons have spoiled the Gulf of Mexico since the Deepwater Horizon blew up April 20, dwarfing the previous largest spill, from the tanker Exxon Valdez in 1989.

"For anyone who cares about national treasures, this is your worst nightmare. And it's not over," said Richard Charter, senior policy adviser on marine programs with Defenders of Wildlife, a conservation group.

BP engineers had begun the "top kill" maneuver to pump dense drilling mud into the renegade well in an attempt to plug it about 1 p.m. Wednesday. The effort was suspended 10 hours later so crews could analyze their work, said Doug Suttles, chief operating officer for BP, which owned the well.

Thursday evening, BP said it had restarted the maneuver and would continue overnight.

Though some of the drilling mud was successfully forced down the well bore — where most of the oil is gushing — a lot of it escaped through leaking pipes connected to the well, Suttles said.

"We have not yet stopped the flow, so the operation has not yet achieved its objective," Suttles said. He said engineers had planned to mix dense rubber balls and other material with the mud to help slow the flow. "Clearly, we need to pump more mudinto the well bore, and that's why we're making some changes." It was unclear Thursday night whether the mix was being used.

Meanwhile, new evidence emerged Thursday that oil was forming massive clouds beneath the surface of the Gulf. A research vessel from the University of South Florida measured hydrocarbons that were concentrated 1,300 feet below the surface, the university announced.

Also Thursday, two congressmen accused BP of omitting critical information from its briefings to Congress about the investigation into the blowout.

Rep. Henry Waxman, D-Calif., and Rep. Bart Stupak, D-Mich., said BP investigators failed to tell lawmakers in a series of briefings this week about the company's decision to use a type of drill casing that is more prone to leaks. The briefing had focused on possible leaks in the well's cement, which was poured by another company.

"This raises the possibility that BP's internal investigation is not examining the consequences of BP's own decisions and conduct," the two lawmakers said in a letter to the company. Waxman chairs the Energy Committee and Stupak is chairman of the Subcommittee on Oversight and Investigations.

BP spokesman Scott Dean said he had not seen the congressmen's letter.

Contributing: Rick Jervis, Alan Levin and the Associated Press