* BP adds another 48 hours to know top kill outcome
* 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 CEO comment, fishing ban, details)
GRAND ISLE/HOUMA, La., May 28 (Reuters) - BP said on Friday it may need two more days to know if its complex maneuver to plug a gushing Gulf of Mexico oil well has worked, while President Barack Obama warned there was no "silver bullet" solution to the biggest oil spill in U.S. history.
Trying to assert leadership in the face of growing criticism over his handling of the spill, Obama toured the Louisiana Gulf coast, where oil has seeped into delicate marshlands and shut down much of the lucrative fishing trade.
BP (BP.L) CEO Tony Hayward flew over the Gulf to where his crew and robots worked on the "top kill" -- the injection of heavy fluids, materials and ultimately cement to seal the well one mile (1.6 km) below the surface.
Hayward said the procedure was making progress choking off the five-week-old leak that has already spewed millions of gallons (liters) of oil into the Gulf.
"We have wrestled it to the ground but we haven't put a bullet in its head yet," Hayward told Reuters as he headed by helicopter to the spill site in the Gulf.
When the top kill began on Wednesday, BP said it would need up to 48 hours to gauge its success. But Hayward extended the timeline another 24-48 hours on Friday. [ID:nN28200489]
He said the top kill's chance of success remained at 60 to 70 percent, although experts put the odds at 50 percent.
BP has called the effort to plug the hole "a "rollercoaster ride," and investors might say the same. BP shares lost 5 percent on Friday, erasing gains made on hopes for a successful top kill.
TAKE A LOOK on the spill [ID:nSPILL]
INSIDER TV: link.reuters.com/wuw64k
On his second visit to the Gulf in the five-week crisis, Obama faced his own steep challenge to convince Americans that he was in command as frustrated Gulf Coast residents loudly criticized federal authorities for being slow to act and offering too little assistance.
"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.
BP 'WORKING FOR THE GOVERNMENT'
The buck may stop with Obama, but the key to stopping the environmental catastrophe lies with BP because the federal government has few tools to work at those depths.
Obama and Hayward have had no contact over the April 20 rig explosion that killed 11 workers and unleashed the gusher, a White House spokesman said. But Hayward made clear that the government is in charge these days.
"We are working for the government," said Hayward. "The government is running it."
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.
As Hayward flew toward the disaster area, the sheen of oil and brown patches of crude and dispersant were visible below some 9 miles (15 km) offshore.
Some 50-60 vessels are in the area working on the BP containment and the shut-off as 12,000 barrels (504,000 gallons/1.9 million liters) to 19,000 barrels (798,000 gallons/3.0 million liters) of oil escape the well per day, according to government estimates.
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.
"I am angry and frustrated I want this thing stopped as fast as I can, as we can," said Hayward, adding, "the reality is that's it's a very challenging technological challenge."
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.
FEW LIKE GOVERNMENT RESPONSE
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."
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.
But Friday brought more bad news for the fishermen. The National Oceanic and Atmospheric Administration said it was extending the closed fishing area to about 25 percent of the federal Gulf waters from around 19 percent before.
Obama inspected oil-trapping booms on a beach and picked up tar balls, but had little contact with the area's residents in the five-hour visit.
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, Kristen Hays, Erwin Seba and Chris Baltimore in Houston, Jeremy Pelofsky in Washington, and Pascal Fletcher and Jane Sutton in Miami; Writing by Mary Milliken, Editing by Sandra Maler)