NEW ORLEANS— BP claimed a key milestone Wednesday in the effort to plug its blown-out well as a government report said much of the spilled oil is gone, heartening officials who have taken heat during the tricky cleanup but leaving some Gulf Coast residents skeptical.

BP PLC reported that mud forced down the well overnight was pushing the crude back down to its source for the first time since the Deepwater Horizon rig exploded off Louisiana on April 20, killing 11 workers.

And a federal report being released Wednesday indicated that only about a quarter of the spilled oil remains in the Gulf and is degrading quickly, with the rest having been contained, cleaned up or otherwise disappeared.

President Barack Obama, while noting that people’s lives “have been turned upside down,” declared in Washington that the operation was “finally close to coming to an end.”

The containment effort isn’t over. Crews performing the so-called “static kill” effort overnight now must decide whether to follow up by pumping cement down the broken wellhead. Federal officials said they won’t declare complete victory until they get into the well from the other end, and that won’t happen for weeks.

“This job will not be complete until we finish the relief well and pump mud and cement in through the bottom,” retired Coast Guard Adm. Thad Allen, the government’s point man on the spill response, said at a news briefing in Washington.

Nearly three-quarters of the oil — more than 152 million gallons — has been collected at the well by a temporary containment cap, been cleaned up or chemically dispersed, or naturally deteriorated, evaporated or dissolved, according to a report by the Interior Department and the National Oceanic and Atmospheric Administration.

“It was captured. It was skimmed. It was burned. It was contained. Mother Nature did her part,” White House energy adviser Carol Browner said on NBC’s “Today” show.

That leaves nearly 53 million gallons in the Gulf. The amount remaining — or washed up on the shore — is still nearly five times the size of the 11 million-gallon Exxon Valdez spill, which wreaked environmental havoc in Alaska in 1989.

About a quarter of the oil evaporated or dissolved in the warm Gulf waters, the same way sugar dissolves in water, federal officials said. Another one-sixth naturally dispersed because of the way it leaked from the well. Another one-sixth was burned, skimmed or dispersed using controversial chemicals.

Nearly 207 million gallons leaked from the well in total, according to government estimates. The cap held back nearly 35 million gallons.

The report’s calculations were based on daily operational reports, estimates by scientists and various analyses by experts. The government acknowledged it made certain assumptions about how oil dissolves in water naturally over time.

Officials, while encouraged by the report, stressed that the fight wasn’t over.

“Less oil on the surface does not mean that there isn’t oil still in the water column or that our beaches and marshes aren’t still at risk,” NOAA Administrator Jane Lubchenco cautioned in a news release.

A Florida State University oceanographer who has long been tracking the spill, and who early on challenged the government’s low estimates of its size, called the report “spin.”

“There’s some science here, but mostly it’s spin, and it breaks my heart to see them do it,” said the oceanographer, Ian McDonald.

“This is an unfortunate report,” he said. “I’m afraid this continues a track record of doubtful information distributed through NOAA.”

Charter boat captain Randy Boggs, of Orange Beach, Ala., said Wednesday he has a hard time believing BP’s claims of success with the static kill and similarly dismissed the idea that only a quarter of the oil remains in the Gulf.

“There are still boats out there every day working, finding turtles with oil on them and seeing grass lines with oil in it,” said Boggs, 45. “Certainly all the oil isn’t accounted for. There are millions of pounds of tar balls and oil on the bottom.”

In the fishing town of Yscloskey, La., crabber Oliver Rudesill, 28, said he has been out of business like most of his buddies, some of whom are doing cleanup for BP instead but are earning about a quarter of what they do fishing.

“As soon as BP gets this oil out of sight, they’ll get it out of mind, and we’ll be left to deal with it alone,” he said Tuesday.

At the entrance to Gulf Islands National Seashore at Pensacola Beach, Fla., Don Allen still wasn’t expecting to sell many snow cones or Italian sausages from his food truck.

“I don’t know where it went if it’s not out there,” said Allen, who had to lay off his son because business has been so slow as tourists abandoned beaches over the summer. “It’s all just numbers, and it has changed so often.”

BP applied nearly 2 million gallons of a chemical dispersant to the oil as it spewed from the well, an attempt to break it into droplets so huge slicks wouldn’t tarnish shorelines and coat marine animals, and to encourage it to degrade more quickly.

In Washington on Wednesday, lawmakers pressed scientists to explain what effects the chemical, whose long-term effects have been questioned, will have on the Gulf’s ecosystem.

Sen. Sheldon Whitehouse, D-R.I., called use of the chemicals a “grand experiment” that didn’t guarantee limited damage from the spill or make clear whether greater harm was possible.

The 75-ton cap placed on the well in July had been keeping the oil bottled up inside over the past three weeks but was considered only a temporary measure. BP and the Coast Guard wanted to plug up the hole with a column of heavy drilling mud and cement to seal it off more securely.

The static kill — also known as bullheading — involved slowly pumping the mud from a ship down lines running to the top of the ruptured well a mile below. A previous, similar effort failed in May when the mud couldn’t overcome the unstemmed flow of oil.

Workers stopped pumping mud in after about eight hours of static kill work and were monitoring the well to ensure it remained stable, BP said.