The slick, which has already come ashore on the Chandeleur Islands off southeast Louisiana, is projected to curl west in the next three days, threatening not only the Mississippi River Delta but also miles of Louisiana coastline to the west of the river.

If the containment dome approach is not successful, other strategies will be explored. But the most dependable solution remains the digging of a nearby relief well, which would allow crews to plug the gushing cavity with heavy liquid.

Mr. Suttles said the drilling of such a well was already at 9,000 feet and proceeding ahead of plan. However, it is still months away from being in a position to stop the leak by intercepting the original well and pumping it full of concrete or other heavy liquid.

Meanwhile, the Obama administration was evaluating proposals for assistance in cleaning up the gulf from the United Nations and the European Union, as well as 14 individual countries, including Sweden, according to the State Department.

Michael Mohr, Sweden’s homeland security liaison at the embassy in Washington, said Saturday that officials in Stockholm were prepared to offer up to three skimming vessels able to collect about 50 tons of oil per hour from the sea and hold about 1,000 tons at a time. The ships could take several weeks to arrive from the Scandinavian peninsula, Mr. Mohr said. “We’re on standby,” he said.

Officials had emphasized for days the difficulty of successfully employing the containment dome. While domes have been used on leaks, they have never been used at such a depth, officials said. BP had anticipated the presence of hydrates, because the oil is mixing with seawater. Hydrates can form when methane bubbling out of the oil comes in contact with water, producing a cage of water molecules surrounding methane molecules.

To prevent that, officials had planned to circulate warm water around the pipe that connected the dome to the ship on the surface. But they had not expected hydrates to accumulate so quickly and at such high concentrations, effectively closing off the dome before the pipe was even attached. “The issue is how to keep them from forming again,” Mr. Suttles said.