British energy giant BP was forced Saturday to back off a plan to contain oil gushing into the Gulf of Mexico with a metal box when icelike crystals encrusted the structure.

Crews have to move the four-story steel dome, which was considered the best short-term hope of containing oil leak from the undersea wellhead, to the side and are trying to find other solutions, BP chief operating officer Doug Suttles said Saturday.

The 100-ton containment box left Port Fourchon, Louisiana, Wednesday on a barge for the oil-well some 52 km off Venice, Louisiana. The box is expected to be placed over the leaking well and collect oil which would be sucked up to a drill ship on the surface.

However, the crystal buildup inside the dome made it too buoyant and clogged it up, said Shuttles. Workers who had carefully lowered the box over the leak nearly a mile below the surface had to lift it and move it to the side. Crews are trying to unplug the box while looking for other solutions.

"I wouldn't say it's failed yet. What I would say is what we attempted to do last night didn't work because these hydrates plugged up the top of the dome," Suttles said. "What we're currently doing, and I suspect it will probably take the next 48 hours or so, is saying, 'Is there a way to overcome this problem?"

As BP continues its oil-containing efforts, oil keeps gushing unchecked into the Gulf of Mexico in what could be the worst US oil spill.

On Saturday, tar balls that are believed to be from the oil spill began washing ashore on Dauphin Island, Alabama, Coast Guard chief warrant officer Adam Wine said.

About a half dozen tar balls, which officials thought came from the spill, had been collected by Saturday afternoon at Dauphin Island, according to Wine.

Last week, a sheen of oil began arriving on land. Though crews have been laying booms, spraying chemical dispersants and setting fire to the slick to try to keep oil from hitting shore, brick-colored oil ribbons are seen gradually approaching Louisiana's shoreline.

About 1.6 million gallons of oil have spilled since the April 20 explosion on the Deepwater Horizon rig, which killed 11 workers. The rig, owned by Swiss-based Transocean and leased by BP, sank April 22, after burning for roughly 36 hours.

The Coast Guard and BP said Saturday about 2.1 million gallons of an oil-water mix had been collected. More than 160 miles of oil-containment boom has been put out and crews have used nearly 275,000 gallons of chemicals to break up the oil on the surface of the Gulf of Mexico.