* BP: will know in next 48 hours if "top kill" worked
* Obama says no "silver bullet" solution to spill
* BP estimates cost of oil spill at $930 million
* BP shares down around 5 percent (Updates with fresh Obama, BP CEO quotes, Louisiana resident)
GRAND ISLE, La., May 28 (Reuters) - BP reported progress on Friday in its struggle to plug its gushing Gulf of Mexico oil well, but President Barack Obama cautioned there was no "silver bullet" solution to the biggest oil spill in U.S. history.
Trying to show leadership in the face of growing criticism over his handling of the spill, Obama on Friday toured the Louisiana Gulf coast, where oil has invaded some of Louisiana's delicate marshlands and closed down the lucrative fishing trade.
Frustrated Gulf Coast residents have loudly criticized federal authorities for being slow to act and offering too little assistance. The White House vigorously disputes this, insisting it has mounted the largest response in history.
"You will not be abandoned. You will not be left behind. We are on your side and we will see this through," Obama said in a televised statement after meeting local and state officials and inspecting the oil spill damage to the coastline.
"I am the president and the buck stops with me," he said.
TAKE A LOOK on the spill [ID:nSPILL]
INSIDER TV: link.reuters.com/wuw64k
BP (BP.L) Chief Executive Tony Hayward told Reuters the so-called top kill procedure, in which heavy "drilling mud" is pumped into the seabed well shaft, was making progress choking off the five-week-old leak that has already spewed millions of gallons (litres) of oil into the Gulf. [ID:nN28200489]
But the success of the operation, never attempted at such depths, was still uncertain and it would be another 48 hours before the company knew if it had worked, he said.
"We have wrestled it to the ground but we haven't put a bullet in its head yet," Hayward told Reuters while aboard a helicopter over the spill site in the Gulf.
He said the top kill's chance of success remained at 60 to 70 percent.
BP views the top kill procedure as its best hope of plugging the well and containing a spill that has tarred its reputation and seen some 25 percent, around $50 billion, wiped off its share price.
The company said on Friday the cost of the disaster so far was $930 million. That figure is sure to multiply with cleanup of the oily mess, which is now larger than the spill from the Exxon Valdez disaster off the Alaskan coast in 1989.
BP shares were down around 5 percent in London amid uncertainty over the success of the effort to plug the well.
Hayward said BP had also injected a "junk shot" of heavier blocking materials, such as shredded rubber and golf balls, into the failed blowout preventer of the ruptured wellhead.
They were due to pump in more heavy fluids later on Friday. The mud has not stopped the oil leak but at times has slowed the flow.
BP began the top kill operation on Wednesday afternoon and then stopped pumping mud overnight to analyze pressure readings. It did not publicize the halt for many hours, drawing fresh accusations it was concealing information from the public. It denied the charges and blamed an oversight.
If the top kill fails, BP says it will try other remedies, such as 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.
Obama said a team of government scientists was exploring contingency plans in case the top kill option failed.
"There are not going to be silver bullets or a lot of perfect answers for some of the challenges that we face," he said. "This is a man-made catastrophe that is still evolving."
The spill is a major challenge for Obama, with opinion polls showing many Americans dissatisfied with the way he has managed the crisis.
Joe Williams, 53, a charter fisherman in Venice, Louisiana, watched Obama on television at the local marina and said he liked it when Obama said "the buck stops" with him.
"If he takes control of it now, that is great because everyone has been passing the buck," he said.
Obama inspected oil-trapping booms at a beach in Port Fourchon, the hub of the Gulf oil industry and one of the areas worst affected by crude coming ashore from the spreading spill.
"Obviously, the concern is that until we stop the flow, we've got problems," said Obama, picking up several tar balls from the beach. An array of oil rigs could be seen offshore.
Friday's trip was 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 a mile (1.6 km) undersea.
His tour comes a day after he vowed to "get this fixed." His 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.
Public approval of the administration's handling of the spill has plummeted, according to the latest Zogby Interactive poll, which surveyed 2,085 people between May 25-27.
Just 16 percent of people surveyed rated the federal government's response as excellent or good, down from 29 percent two weeks ago.
(Additional reporting by Ed Stoddard in Venice, Louisiana, Tom Bergin in Houston, Kristen Hays in Houston, Jeremy Pelofsky, Matt Spetalnick, Alister Bull and Caren Bohan in Washington, and Pascal Fletcher and Jane Sutton in Miami; Writing by Pascal Fletcher and Ross Colvin; Editing by Doina Chiacu and Sandra Maler)