Barack Obama warned of a "massive and potentially unprecedented environmental disaster" as a wrecked BP oil well spewed a widening and deadly slick towards delicate wetlands and wildlife.
The US president said it could take many days to stop the flow from the mile-deep sunken Deepwater Horizon rig in the Gulf of Mexico.
Mr Obama rushed to southern Louisiana to see the efforts against the oil gush as cabinet members insisted he was doing everything he could.
Then he took a helicopter ride over the water to view the 30-mile oil slick caused by as much as 210,000 gallons of crude gushing into the Gulf each day.
The spill threatens not only the environment but also the region's abundant fishing industry, which Mr Obama called "the heartbeat of the region's economic life".
It appeared little could be done to stem the oil flow, which was also drifting towards the beaches of neighbouring Mississippi and further east along the Florida Panhandle.
Mr Obama said the slick was nine miles off the coast of south-eastern Louisiana.
BP chairman Lamar McKay raised faint hope that the spill might be stopped more quickly by lowering a hastily manufactured dome to the ruptured wellhead in the next six to eight days, containing the oil and then pumping it to the surface.
Such a procedure has been used in some well blowouts but never at the mile-deep waters of this disaster.
The leaking well was not only an ecological disaster but a potential political hazard, as well, depending on how the public judges the Obama administration's response.
In 2005, President George Bush stumbled in dealing with Hurricane Katrina in the Gulf and left the impression of a president distant from immense suffering. His presidency never recovered.
Mr Obama vowed that his administration, while doing all it could to mitigate the disaster, would require well owner BP America to bear all costs.
"Your government will do whatever it takes for as long as it takes to stop this crisis," he said. "BP is responsible for this leak. BP will be paying the bill." Mr Obama said after a US Coast Guard briefing in Venice, Louisiana.
The president stood before cameras in a heavy rain, water dripping from his face.
He also stopped to talk to six fishermen and said the challenge was "how do we plug this hole?". After that, he said, protecting the estuaries would be the next priority.
"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," Mr Obama said.
After arriving in New Orleans, the president shunned helicopter travel because of a threat of tornadoes and drove to Venice to tour a close-to-the-water staging area where the government and BP were trying to keep the slick from causing even more damage.
Homeland security secretary Janet Napolitano said com-parisons with Katrina were "a total mischaracterisation" and that the government had taken an "all hands on deck" approach from the beginning.