| VENICE, La.
VENICE, La. BP reported some progress on Friday in its struggle to shut off its gushing Gulf of Mexico oil well, and President Barack Obama headed to the coast to assert control in the fight against the largest oil spill in U.S. history.
BP Chief Executive Tony Hayward said the so-called top kill procedure, in which heavy drilling "mud" is pumped into the seabed well shaft, was having some success in choking off the leak that has already spewed millions of gallons (liters) of oil into the Gulf.
But the success of the operation, never attempted at such depths, was still uncertain and it could be another 48 hours before it would be known whether it was successful, he said.
"We don't know whether we will be able to overcome the well," Hayward told NBC's "Today" show. The British-based energy giant was maintaining its assessment that the "top kill" plugging operation had a 60 to 70 percent chance of success.
The spill is a major challenge for Obama, who will visit the Louisiana coast where sticky oil has permeated wetlands, closed down a lucrative fishing trade and angered locals whose communities are still recovering from Hurricane Katrina in 2005.
Opinion polls show many Americans are dissatisfied with Obama's handling of the five-week-old crisis. He was on the defensive at a news conference on Thursday, rebutting criticism that his administration had been too slow to act and too quick to believe what it was being told by BP.
Hayward said BP engineers had injected a "junk shot" of heavier blocking materials -- such as pieces of rubber -- into the failed blowout preventer of the ruptured wellhead.
Later on Friday, they would pump in more heavy fluids -- all part of the top kill procedure.
"We have some indications of partial bridging which is good news," he told CNN. "I think it's probably 48 hours before we have a conclusive view," he added.
Thad Allen, a Coast Guard admiral who is leading the oil spill response, told ABC: "We're very encouraged by the fact that they're able to push the mud down. The real question is can we sustain it, and that will be the critical issue going through the next 12 to 18 hours."
BP shares were down about 4 percent in London amid uncertainty over the success of the effort to plug the well.
BP said on Friday the cost of the disaster so far was $930 million. The cost is sure to multiply with cleanup of the spill, which has now surpassed the Exxon Valdez disaster off the Alaska coast in 1989.
"This is clearly an environmental catastrophe, there are no two ways about it," Hayward told CNN, reversing previous comments in which he had minimized the spill's ecological impact.
POLITICAL CHALLENGE FOR OBAMA
Friday's trip will be Obama's second to the Gulf in the more than five weeks since a rig explosion killed 11 workers and unleashed the oil from a well head one mile (1.6 km) down.
His tour comes a day after he vowed to "get this fixed" as criticism swelled over what many Americans see as a slow government response to one of the country's biggest environmental catastrophes.
Obama's predecessor, George W. Bush, was slammed for his administration's handling of Hurricane Katrina and Obama is anxious to avoid comparisons.
The spill could turn into a major political liability for Obama before November elections that are widely expected to erode his Democratic Party's control of the U.S. Congress.
At the White House on Thursday, Obama took ownership of the crisis in a shift from earlier statements in which he placed all the responsibility for cleaning up the mess on the shoulders of BP.
But, however much he seeks to assert control, the federal government lacks the tools and technology to solve the deep-sea disaster and depends on BP to find the way to staunch the flow.
If the top kill fails, BP says it will try other remedies, such a second attempt at containing the oil so it can be transported by pipe to a ship at the water's surface or placing a new blowout preventer atop the failed one.
It is also drilling two relief wells that will stop the flow but those will take several weeks to complete.
Obama flew to Louisiana from Chicago, where he is spending a holiday weekend, on Friday morning. He was due to be briefed at the Coast Guard station on Grand Isle before visiting a beach and helping set up an oil-trapping boom.
The scale of the spill expanded hugely with new government calculations on Thursday that put the flow rate from the ruptured well at as much as four or five times BP's estimate of 5,000 barrels (210,000 gallons/795,000 liters) a day.
In the Louisiana wetlands, scientists showed where oil washed into wild cane fields, discoloring the base of green cane and reeds and piercing the air with its pungent smell.
Many of these small islands of wetlands were surrounded by the white protective boom that has been laid out to prevent the oil from seeping in but it was clearly being breached.
"Each of these islands has been fouled," said Ian MacDoland, a professor of oceanography at Florida State University, as he surveyed the scene.
(Additional reporting by Jeremy Pelofsky and Matt Spetalnick in Washington and Pascal Fletcher in Miami; Writing by Pascal Fletcher and Ross Colvin; Editing by Doina Chiacu)