BP's plan to contain the Gulf of Mexico oil leak with a big metal box was fraught with unknowns and potential problems, it was reported on Sunday.

Gas hydrates -- slushy crystal formations of natural gas and water that form under pressure -- had plugged the opening at the top of the box that is supposed to funnel the oil geyser into pipes connected to a ship, the Los Angeles Times said, quoting BP officials.

"I wouldn't say it's failed yet. But what we attempted to do last night didn't work," said Doug Suttles, BP's chief operating officer.

BP had spent Friday lowering the specially built containment dome, which resembles a squat, four-story building, from a ship to the gulf floor nearly a mile (1.6 kilometer) below. It succeeded in placing the device over the main leak, a crumpled riser pipe that broke during the Deepwater Horizon rig explosion April 20.

But not long after the structure was in place, hydrates formed at its top. A crane positioned on a ship then moved it about 600 feet from the leak, where it is resting on the seafloor while BP tries to devise a remedy, the report said. Company engineers had anticipated that hydrates clogging the pipe system would be their biggest challenge, but they thought the hole at the top of the metal structure was too large to be blocked.

The hydrates, which fizz when brought to the surface, can be easily dislodged by raising the dome. But BP teams have to figure out how to prevent them from forming again at the leak's 5,000- foot depth, according to the report.

They are trying to figure out whether there is "a way to overcome this problem," Suttles said.

Containment devices have been used before, but never at such deep depths. "We're doing things we've never done before and it's difficult to know if they'll work," Suttles said.

Meanwhile, tar balls were beginning to wash up on Alabama's Dauphin Island, a Gulf Coast playground, the report quoted the Coast Guard as saying.

If the black gobs prove to be oil, it would be the easternmost landfall from the BP oil slick, which so far has proved to be more of a threat to Louisiana and Mississippi, the report said.

A half-dozen balls had been found, and the substance was being analyzed but is believed to be part of the oil spill, the report quoted Alabama officials as saying.

Since the rig explosion April 20, about 200,000 thousands of gallons of crude has been spewing into the Gulf each day.