BP began critical testing on Tuesday of its "top kill" plan to plug the gushing Gulf of Mexico oil well while President Barack Obama vented frustration over the energy giant's inability to contain what is likely the largest
U.S. oil spill ever.

Five weeks into an environmental disaster with no end in sight, Obama has told aides to "plug the damn hole" and plans to make his second visit to the Louisiana Gulf coast on Friday.

But before he heads to the disaster zone, Obama is expected to send a message on Thursday to the oil industry of tougher safety requirements for offshore drilling rigs, an official said.

BP admitted it may have made a "fundamental mistake" in continuing work on the deepwater rig despite signs of excess pressure in the hours before the April 20 explosion that killed 11 crew members, a memo Tuesday from two congressmen said.

Mindful of growing public criticism of its oil spill response, the government stressed its involvement and its authority over BP by sending top scientists like Energy Secretary Steven Chu into the planning room with BP.

"We have our scientists in the room looking at what is being proposed, at what is the next step. And, obviously, these steps are taken only after we reach agreement," Carol Browner, Obama's adviser on energy and climate change, told CNN.

On an Air Force One flight to California, Obama was updated on BP's top kill plan in a phone call from Chu, who gave him technical and scientific assessment of the plan and what the next steps will be if this attempt fails to stop the leak, the White House said.

As the gloppy mousse-like oil oozes into critical wildlife habitats and fishing grounds, the U.S. government has few tools or technology to tackle the undersea well blow-out triggered by a rig explosion April 20 that killed 11 crew members. It mostly depends on BP to work miracles one mile below the surface.

The government-Big Oil relationship came under new scrutiny on Tuesday with a damning official report that points to a "cozy relationship" between the industry and the agency meant to regulate it.

Interior Secretary Ken Salazar ordered an investigation into whether the oil rig involved in the spill was properly monitored by the Minerals Management Service.

"This deeply disturbing report is further evidence of the cozy relationship between some elements of MMS and the oil and gas industry," said Salazar in a statement, who has tried to reform the MMS since taking over in 2009.

The years under investigation fall under the previous administration of President George W. Bush and his vice president, Dick Cheney, who were widely seen as friendly to big oil companies.

With the accident and failures to cap the well, BP has seen around 25%, almost $50 billion, wiped off its market value, and London shares lost another 1.6% Tuesday.

Adding to its woes was an oil spill of several thousand barrels on the Trans Alaska Pipeline, operated by a consortium in which BP is a major shareholder.

Top Kill Tests Well

Equipped with underwater robots, BP engineers plan on Wednesday to inject heavy drilling fluids into the mile- (1.6 km) deep well, a complex maneuver known as "top kill" that has never been attempted before at such depths.

Before they try to seal the well, they pumped so-called "mud" into the well head on Tuesday to gauge if the well could be damaged at high pressure and augment the leak.

Industry experts told the Reuters Global Energy Summit that the top kill plan is "doable" and has a 50-50 percent chance of working, while playing down concerns of a bigger leak.

BP does have other options if the top kill fails, including the installation of a new dome and a new blow-out preventer over the old one that failed in the rig explosion.

Top Kill Via Video

Analysts say the oil spill could become more of a political liability for Obama ahead of November elections that are widely expected to erode his Democratic party's control of the U.S. Congress.

A CBS News poll released Tuesday showed 70% of Americans disapprove of BP's handling of the disaster, but 45% give the Obama administration a negative rating despite his administration's efforts to show it is on top of the crisis.

Obama cited the spill when he met Republicans on Capitol Hill on Tuesday and urged them to work with him to pass climate change legislation that aims to reduce U.S. dependence on foreign oil and better protect the environment.

With oil and tar balls from the spill now soiling more than 70 miles (113 km) of Louisiana's 400-mile (644 km) coastline, the U.S. government has declared a "fishery disaster" in the waters off Louisiana, Mississippi and Alabama, making those states eligible for special federal assistance.

Commercial fishing, shrimping and oyster harvests have been shut down for weeks along much of the U.S. Gulf Coast, home to a $6.5 billion seafood industry. Louisiana's industry alone accounts for up to 40% of the U.S. seafood supply and more than 27,000 jobs.

BP has estimated that about 5,000 barrels (210,000 gallons/795,000 liters) have been leaking every day, although some scientists have given much higher numbers for the size of the leak - up to 20 times more.

One way of monitoring BP operations is the live video feed of the leak, which BP said it might turn off during the top kill procedure. But an administration official said BP agreed to make the live feed of the top-kill attempt available at the request of Obama and the National Incident Center.