BP has hit a snag in its efforts to lower a steel dome to trap oil gushing from an oil well in the Gulf of Mexico.
Chief operating officer Doug Suttles says it could take two days to fix the problem.
BP says flammable hydrate formations were encountered while the four-storey structure was being lowered onto the leak on the sea floor.
Hydrates are flammable crystals formed when gas combines with water under certain pressure and temperatures.
The crystals are lighter than water, making the dome buoyant. The dome is now on the seafloor while engineers try to solve the problem.
The dome is designed to fit over the biggest of two leaks nearly 1.6km below the surface of the Gulf and funnel the oil to a tanker above.
Oil has been leaking from the well 80km off Louisiana, since an explosion destroyed the Deepwater Horizon rig, killing 11 men. It sank on 22 April.
Protection measures
The Coast Guard and BP said on Saturday that about 2.1 million gallons of an oil-water mix had been collected, about 10% of which was oil.
More than 257km of booms had been put out and crews have used nearly 275,000 gallons of dispersants.
