LOS ANGELES - President Obama announced Thursday that he would return to the Gulf of Mexico, as a mechanical cap that may finally stem the river of BP crude was being maneuvered onto the gushing well pipe.

The visit Friday would be Obama's third since the April 20 explosion on the Deepwater Horizon killed 11 people and unleashed what has become the largest oil spill in U.S. history. The administration, which in the last week has assumed a more demanding stance toward BP, on Thursday sent the company a preliminary cleanup bill for $69 million, a small portion of the estimated $1 billion cost of the spill response so far.

Anywhere from 21 million to 46 million gallons of oil has spewed into the gulf, according to government estimates.

The containment operation moved ahead Thursday after undersea robots succeeded in shearing a leaking riser pipe connected to the blown-out wellhead. Facing scorn and anger over his company's inability to wrest control of the runaway oil well, BP chief executive officer Tony Hayward called Thursday's events an "important milestone."

But as has become customary with each halting step to contain the spill, there were caveats. Hayward said it was too early to tell how much of the oil spout - gushing 500,000 to 800,000 gallons a day - would be captured by the cap and sent to a ship.

Engineers had predicted that slicing away the mangled riser pipe would increase the well gusher by as much as 20 percent because kinks in the pipe would no longer restrict the well flow. As the cap descended Thursday afternoon, a bubbling black cloud of oil continuously erupted from the sheared pipe stub like a mini undersea volcano.

Live video Thursday night showed an inverted funnel-like cap slightly wider than the severed pipe. However, the gushing oil made it very difficult to tell if the cap was fitting well. BP spokesman Toby Odone said he had no information on whether the cap had been attached.

"We recognize this is just the beginning," Hayward said of the capping operation. He promised that his company would work as long as it takes to clean up the spill and restore the livelihood and way of life to gulf residents.

Unable to make a finely calibrated cut when an industrial-size diamond-wire saw became stuck, engineers used giant hydraulic shears to cleave the pipe, leaving a rougher edge that will be more difficult to seal and potentially allow more oil to escape.

"This is an irregular cut. It will be a bit more challenging," Coast Guard Adm. Thad Allen, the national incident commander, said at a briefing Thursday morning.

Crews will use methanol to try to prevent crystals from forming on the inside of the cap. At this depth a mile underwater, the near-freezing temperatures can cause a buildup of hydrates, which foiled the attempt to place a 100-ton, four-story dome over the leak about a month ago.

Since last week's futile attempt to plug the deep-sea well with a long-planned "top kill" procedure, federal officials have emphasized that the final fix is still several months away. The completion of two relief wells will allow engineers to pump cement into the bottom of the damaged well, permanently sealing it.

BP officials say that the first relief well already extends more than 12,000 below sea level, about halfway to the target, but because drilling gets slower as a well gets deeper, it is not expected to be finished before August. The second well was started later and is not as deep.

The wells cost about $100 million each and are being drilled from rigs owned by Transocean, the company that owned the Deepwater Horizon.

The work could be stalled by hurricanes or by equipment or drilling problems, and the wells might initially miss the target, causing delays as the drill bits have to be backed up and redirected. But BP officials and outside experts say that the relief wells will work. It is a matter of when, they say, not if.

"This is the answer," said Walt Warchol, a retired drilling engineer in Houston. "It's just going to take some time."

BP also launched a national ad campaign Thursday, apologizing for the spill and assuring the public that it would pay for the cleanup and all legitimate damages. "The gulf spill is a tragedy that never should have happened," Hayward said in the TV spot.

Hayward's appeal contrasted with the flippant tone of a remark he made to reporters Sunday, saying that he "would like his life back." Fishermen and other Gulf Coast residents expressed outrage at the comment. In a mea culpa posted on BP's Facebook page Wednesday, Hayward acknowledged that he "made a hurtful and thoughtless comment."

"When I read that recently, I was appalled," he said. "I apologize, especially to the families of the 11 men who lost their lives in this tragic accident."

BP may have a long way to go in varnishing its image. Protest groups have planned a week of anti-BP demonstrations in 50 cities. Thursday, the credit-rating firms Moody's Investors Service and Fitch Ratings lowered their assessments of BP's long-term debt.

Fitch cut BP's debt rating to AA from AA-plus, citing the potential for civil and criminal charges, as well as increasing "risks to both BP's business and financial profile." Fitch estimated that the company could spend as much as $3 billion on cleanup this year.

The portion of federal gulf waters closed to fishing is approaching 40 percent as strands of the slick move toward Florida. There were reports of oily substances and tar balls at the Florida Keys.

The damage to the environment was chilling on East Grand Terre Island along the Louisiana coast, where workers found birds coated in thick, black goo. Images shot by an Associated Press photographer show pelicans drenched in thick oil, flailing in the surf.

This article contains information from the Associated Press and the New York Times News Service.