President Barack Obama stepped up the battle against a massive slick lapping the Gulf of Mexico coast, vowing never to abandon those imperiled by the worst US oil spill.
As BP pleaded for patience to allow time for its risky, complex "top kill" to work and plug the massive leak, Obama pledged "to continue to do whatever it takes to help Americans whose livelihoods have been upended by the spill."
The US president, clad in hiking boots and with his sleeves rolled up, ordered the number of workers feverishly trying to contain and clean up the spill along the southern US coastline to be tripled.
He toured an oil-slicked Louisiana beach, picking up tar balls to examine them, as he outlined his administration's "historic response" to the disaster which has spewed millions of gallons of oil into the Gulf of Mexico.
His second trip to the region since an April 20 explosion tore through the Deepwater Horizon rig, 50 miles (80 kilometers) off shore, came as experts and residents hold their breath, hoping BP can stop the oil flowing from a fractured pipe.
Government scientists estimate some 12,000 to 19,000 barrels of crude a day have been gushing into the Gulf since the rig sank two days after the blast which killed 11 workers.
"I think the key element here is to exercise patience," said BP's chief operating officer Doug Suttles, adding the operation would last another 24 to 48 hours.
The British energy giant is using robotic submarines to pump heavy drilling fluids down the wellhead, hoping to drown the leak long enough to allow engineers to then seal it with cement.
"We'll have periods where we're pumping. We'll have periods where we're monitoring results of that pumping. We'll have periods where we actually pump in this, what we call junk," Suttles said, seeking to allay concerns over why BP had stopped the pumping several times since it began on Wednesday.
The disaster has already closed stretches of coastal fishing waters, endangering livelihoods which are also dependent on tourism, and threatening a catastrophe for Louisiana marshes, home to many rare species.
"To the people of the Gulf Coast, I know you've weathered your fair share of trials and tragedy," Obama said, in a reference to the 2005 Hurricane Katrina which triggered a botched response by the former Bush administration.
"I'm here to tell you that you're not alone. You will not be abandoned. You will not be left behind," Obama vowed. "We are on your side and we will see this through."
He said 20,000 people had already been deployed to contain and clean up the spill, but that he had ordered Homeland Security Secretary Janet Napolitano and US Coast Guard chief Admiral Thad Allen to "triple the manpower in places where oil has hit the shore or is within 24 hours of impact."
Allen, who has been charged with overseeing the government's response, said initial signs suggested BP's "top kill" was succeeding.
"They have been able to push the hydrocarbons down with the mud. The real challenge is to put enough into the well to keep the pressure where they can put a cement plug over the top," he said on ABC's "Good Morning America."
Government data released Thursday would mean between 18.6 million gallons and 29.5 million gallons of oil have poured into the Gulf -- far more than the roughly 11 million gallons of crude spilled in the 1989 Exxon Valdez disaster.
Amid the environmental catastrophe, there were also growing fears for the health of cleanup workers exposed to the oil and chemical dispersants.
Two more crewmen aboard ships helping burn off surface oil were evacuated to hospital late Friday after falling ill, a day after the US Coast Guard announced that seven workers were evacuated for medical emergencies.
Obama said three million feet (910,000 meters) of hard boom had already been deployed in an effort to stop the oil spill reaching wetlands and beaches. But he admitted "there's a limited amount" available.
"We're going to try to get more boom manufactured, but that may take some time," he said.
The commander of a federal research ship who has spent five days out at sea on the edges of the slick said a heavy smell of oil hung over the area.
"It's a strong smell out there," said commander Shepard Smith of the Thomas Jefferson, a 204-foot survey vessel. "It smells like freshly creosoted railroad ties."
BP said Friday the oil spill had cost the firm about 930 million dollars, while the company's market value has also dropped by billions.