BP reported some progress on Friday in its struggle to shut off its gushing deepwater Gulf of Mexico oil well, and President Barack Obama was set to assert control with a visit to coastal areas threatened by the largest oil spill in U.S. history.

BP Chief Executive Tony Hayward said a “top kill” attempt that started on Wednesday to plug the ruptured seabed well had had some success in keeping oil and gas down in the bore. But the final outcome was still uncertain and it could be another 48 hours before it would be known whether it was successful.

“We don’t know whether we will be able to overcome the well,” he told NBC’s “Today Show”. The British-based energy giant was maintaining its assessment that the “top kill” plugging operation had a 60-70 percent chance of success.

Rising public anger and frustration over the uncontrolled spill has made it 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 still on the mend from Hurricane Katrina in 2005.

Appearing on several U.S. TV morning news shows, 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 also pump in more heavy drilling “mud” — all part of the top kill procedure being attempted.

“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’s “Good Morning America,” the next 12 to 18 hours would be “very critical”

BP shares were down around 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, up from a $760 million estimate on Monday. The cost is sure to multiply with clean-up 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 by him in which he had predicted the ecological impact from the spill would be small.

POLITICAL CHALLENGE FOR OBAMA

Friday’s trip will be Obama’s second visit 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. [nN27148649]

Obama’s predecessor, George W. Bush, was slammed for his administration’s handling of Hurricane Katrina, and Obama is anxious to avoid comparisons.

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 stanch the flow. Relations between the two camps have been strained as Washington put the blame squarely on the London-based company.

If top kill fails, BP said it will immediately try other remedies, such as containing the oil so it can be transported by pipe to a drillship 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.

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.

The U.S. Geological Survey now estimates that the flow ranges from 12,000 barrels (504,000 gallons/1.9 million liters) to 25,000 barrels (1.05 million gallons/3.97 million liters ) per day. The team’s best estimate is 12,000 to 19,000 barrels per 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 in Washington and Pascal Fletcher in Miami; Writing by Mary Milliken and Pascal Fletcher; Editing by Frances Kerry)