For the first time since an explosion on a drilling rig 15 days ago left an undersea well spewing crude oil into the Gulf of Mexico, engineers succeeded in shutting off one of the three leaks from the damaged well late Tuesday night, a spokesman for BP said on Wednesday morning.

Though by itself the move was not expected to reduce the amount of oil being released — estimated at 210,000 gallons a day — it “does enable to us to make progress, to winnow down the focus from three leaks to two,” said the spokesman, John Curry.

Submersible robots, controlled remotely from a ship on the surface, were able to place a specially designed valve over the end of a leaking drill pipe lying on the sea floor in water about 5,000 feet deep, and stop oil from escaping at that point, Mr. Curry said. The company had been trying since Monday to place the specially designed valve but had been hampered by rough seas and high winds that diminished on Tuesday.

With one valve shut, BP is now turning its attention to capping the worst of the leaks. Crews have loaded a 98-ton, four-story structure called a containment dome onto a barge, and are expected to tow it at midday Wednesday to the site of the spill, a 12-hour trip.

The plan is to lower the dome to the sea floor and place it over the leak, capturing the gushing oil and funneling it up to a rig waiting at the surface. That is expected to be operational by the end of the week, BP officials said.

Weather conditions continued to look promising on Wednesday, allowing recovery crews to move forward on several fronts to control the spread of the oil slick.

Mr. Curry said crews were preparing to conduct a “controlled burn” of some of the oil at sea, and that skimming boats were out in other areas to corral the thick oil from the surface of the water. Aircraft were also expected to resume dropping dispersants from overhead.

The report of progress in containing the leak came as BP, which is responsible for cleaning up the vast oil spill resulting from the fatal explosion and fire that destroyed the rig it was leasing, acknowledged that the flow of oil could become vastly larger than initially estimated.

In a closed-door briefing for members of Congress, a senior BP executive conceded Tuesday that the ruptured oil well could conceivably spill as much as 60,000 barrels a day of oil, more than 10 times the estimate of the current flow.

The scope of the problem has grown drastically since the rig, the Deepwater Horizon, sank into the gulf. Now, the discussion with BP on Capitol Hill is certain to intensify pressure on the company, which is facing a crisis similar to what the Toyota Motor Company had with uncontrolled acceleration — despite its efforts to control the damage to its reputation as a corporate citizen, the problem may be worsening.

Amid growing uncertainty about the extent of the leak, and when it might be stanched, pressure on BP intensified on multiple fronts Tuesday, from increasingly frustrated residents of the Gulf Coast to federal, state and local officials demanding more from the company.

The company considered a broad advertising campaign, but top BP executives rejected the idea before planning even started. “In our view, the big glossy expressions of regret don’t have a lot of credibility,” said Andrew Gowers, a BP spokesman.

Instead, the company has dispatched executives to hold town meetings in the affected region, and it has turned to lower-profile social media outlets to trumpet its cleanup efforts and moves to organize volunteers.