President Barack Obama is briefed by National Incident Commander U.S. Coast Guard Commandant Adm. Thad Allen in Venice, La., as he visits the Gulf Coast region affected by the BP (British Petroleum) oil well spill, Sunday, May 2, 2010.

President Barack Obama says an expanding oil slick in the Gulf of Mexico is a massive and unprecedented environmental disaster that will require a relentless, coordinated effort to overcome. The president traveled to the southern state of Louisiana on Sunday to meet with Coast Guard officials and other responders who are trying to contain the spill.

A rain-soaked President Obama delivered a somber message from a Coast Guard station on Louisiana's threatened coastline. "We are dealing with a massive and potentially-unprecedented environmental disaster.  The oil that is still leaking from the well could seriously damage the economy and the environment of our Gulf states, and it could extend for a long time.  It could jeopardize the livelihoods of thousands of Americans who call this place home," he said.

Accompanied by Louisiana Governor Bobby Jindal, the president was briefed by Coast Guard commanders and other officials taking part in the disaster response.

Mr. Obama said that even while hoping for the best, the federal government is prepared for the worst, if the spill from the underwater well continues.  He promised a full investigation of the disaster, and said that petroleum giant BP will be held responsible for the accident.  But for now, Mr. Obama said, there is work to be done. "Every American affected by this spill should know this:  Your government will do whatever it takes for as long as it takes to stop this crisis," he said.

An explosion nearly two weeks ago aboard the now-sunken oil rig killed 11 workers.  The U.S. Coast Guard says millions of liters of oil have since spilled into the Gulf of Mexico.

Appearing on ABC's "This Week" program, the chairman of BP America, Lamar McKay, said there is no way to know how much oil is flowing into the Gulf of Mexico. "I do not know the volume.  The volume is uncertain.  There is a large uncertainty range around the [estimate of] 5000 barrels.  Our spill response is designed to take that uncertainty into account," he said.

A mechanism on the rig designed to stop the flow of oil in the event of a spill malfunctioned, leaving BP now trying to prepare a dome that the company intends to place over the well.

Federal officials say stopping the oil flow could take weeks or even months.

U.S. Interior Secretary Ken Salazar also appeared on "This Week." "Hopefully something will happen [to stop the spill]. Best case scenario - it gets stopped today. Worst case scenario - this thing could keep going on for 90 days," he said.

The oil slick threatens ever-larger swaths of the U.S. coastline, placing sensitive marine ecosystems in peril.  If uncontained, experts say oil could saturate beaches and marine habitats as far away as Florida, devastating fishing and tourism.

The disaster comes as Obama administration attempts to chart a course toward U.S. energy independence and less reliance on fossil fuels. President Obama endorsed off-shore drilling as part of a comprehensive strategy, but he has halted new drilling projects until investigators determine the cause of the BP accident.