Officials for BP on Saturday encountered a significant setback in their efforts to attach a containment dome over a leaking well on the seabed of the Gulf of Mexico, forcing them to move the dome aside while they find another method to cap the crude oil flowing into the Gulf since April 20.

Officials discovered that gas hydrates, ice-like crystals lighter than water, had built up inside the 100-ton metal container. The hydrates threatened to make the dome buoyant, and they also plugged up the top of the dome, preventing it from being effective.

“I wouldn’t say it has failed yet,” Doug Suttles, BP’s chief operating officer, said at a news conference in Robert, La. “What we attempted to do last night hasn’t worked.”

As a consequence, crews had to lift the dome off the well and place it on the seabed.

BP officials said they had anticipated a problem with hydration — but not this soon in the operation. Since last week they had been cautioning that this type of procedure had never before been tried at 5,000 feet below the surface.

BP has struggled to find any method to stem the majority of the oil, leaking at least 5,000 gallons barrels — roughly 210,000 gallons — each day.

For now, they have put the dome 650 feet to the side of the leaking well, “while we evaluate options,” Mr. Suttles said.

The containment dome was supposed to be the largest-scale method to cap the majority of the oil flow so far. Other efforts continued on Saturday, as BP said that the drilling a relief well, which would be able to collect the oil at one source of the leak, had reached 9,000 feet.

Weather prevented crews from doing a controlled burn of some of the oil, as they had done successfully on Friday, but they were still able to lay protective boom, said Rear Adm. Mary Landry of the Coast Guard.