He also acknowledged that the spill could be disastrous for the region's economy, especially for the fishing industry.
"We're going to do everything in our power to protect our natural resources, compensate those who have been harmed, rebuild what has been damaged and help this region persevere like it has done so many times before," Obama promised.
Fishing shut down
US authorities have shut down commercial and recreational fishing in the Gulf Coast area for 10 days and fishermen like 61-year-old Jimmy Rowell in Mississippi, said "if this oil comes ashore, it's just over for us".
Louisiana accounts for an estimated one-third of the country's total oyster output, and the Gulf of Mexico is prime spawning waters for fish, shrimp and crabs, as well as a major stop for migratory birds.
|US officials have closed fishing near the coast for 10 days [AFP]
While the government has mobilised masses of equipment to scoop up, burn and block the oil from moving ashore, in reality, oil companies and the government appear powerless to prevent the damage from a well gushing an estimated 800,000 litres of crude each day.
Ken Salazar, the interior secretary, admitted on Sunday that it was "a very grave scenario".
"You're looking at potentially 90 days before you ultimately get to what is the ultimate solution," he said, alluding to a plan to drill relief deep into the ocean floor, where the oil could be diverted.
Aside from the ecological and economical impact, there could be potential political fallout as well, depending on how the public judges the Obama administration's response.
George Bush, Obama's predecessor, stumbled in dealing with Hurricane Katrina in the Gulf in 2005 and left the impression of a president distant from immense suffering - an impression his presidency never recovered from.
Obama administration officials say they were on top of the accident from the first day but a declaration of "national significance" - opening the way for greater government involvement - was not made for nine days.
Amid increasing criticism of the response of the government and oil giant BP, Janet Napolitano, the homeland security secretary, said any comparison between the ruptured BP oil well and Katrina was "a total mischaracterisation".
Obama, too, defended his government's response to the potential disaster and pledged to spare no effort or resources to "clean up whatever damage it may cause".
"The federal government has launched and co-ordinated an all hands on deck relentless response to this crisis from day one after the explosion on the drilling rig," he said in the town of Venice after a coast guard briefing.
"From day one we have prepared and planned for the worst even if we had hoped for the best, while we have prepared and reacted aggressively, we are not going to rest or be satisfied until the leak has stopped at the source."
Al Jazeera's Sebastian Walker, reporting from Venice, said that residents had come out to see Obama and appeal for something to be done to help them.
"There are a lot of people here who are extremely concerned about their livelihoods," he said.
"People are extremely concerned about what is going to happen next and concerned that the response so far has not been particularly fast."
An investigation is under way into the cause of the April 20 well explosion that killed 11 people, but the president made it clear on Sunday that his administration would require well owner BP to bear all costs.
"BP is responsible for this leak; BP will be paying the bill," Obama said.
Lamar McKay, BP's chairman, has defended his company's safety record, blaming the disaster on "a failed piece of equipment'' and saying that efforts to activate a "blowout preventer" have been "like doing open-heart surgery at 5,000 feet [1,525 metres]".
McKay raised faint hope on Sunday that the spill might be stopped by lowering a hastily manufactured dome to the ruptured wellhead a mile deep in the next six to eight days, containing the oil and then pumping it to the surface.
But such a procedure, while used in some well blowouts before, has never been attempted at the depth of this disaster.
Bad weather has also been hampering the emergency efforts.
Recovery workers have been deploying miles of inflatable booms hoping to contain the oil, but strong winds and heavy seas have caused many of the barriers to break loose.
Bob Riley, the governor of Alabama state, said 80 per cent of the booms laid down off his state over the previous three days had broken.
Chemical dispersant is also being used underwater in an attempt to tackle the oil as it leaves the broken riser, but satellite images indicate the slick has tripled in size in just two days, suggesting the oil could be pouring out faster than before.
Government data on Sunday showed that the thickest part of the huge slick had turned northward towards the remote Chandeleur Islands, a chain of uninhabited islets in eastern Louisiana that is an important wildlife area.
The coast guard and BP have said it is nearly impossible to know exactly how much oil has gushed out since the blast nearly two weeks ago, but it has been estimated at nearly 800,000 litres a day.
At that rate, the spill would eclipse the 1989 Exxon Valdez incident as the worst US oil disaster in history, in a matter of weeks.
That disaster spilled an estimated 40.9 million litres of crude oil into the previously environmentally pristine waters of Prince William Sound, devastating local wildlife.