The admiral said that a few years ago he led a training exercise on dealing with a major spill of oil resulting from a blown out well. But that exercise was held in far shallower waters and the simulation prepared for an oil slick that was more compact in character.

“No one anticipated that this would spread out across such an area” and involve “hundreds of thousands” of patches, he said. As a result, the Coast Guard has had to recruit a flotilla of volunteers, hundreds of boats that will be equipped with booms and skimming devices, to clean up the scattered oil. But even skimming operations have to be adapted to the depth of ocean and matched to the kinds of vessels available.

The operation, Admiral Allen said, was “taxing our resources.”

In a statement on its Web site, BP said one of four vents on its containment cap had been closed. When the cap was first lowered last week, the company said it hoped to close all four vents gradually to increase the collection rate.

But a technician involved in the effort said Monday that it was unlikely that the other three vents would be closed  both because the surface ship, the Discoverer Enterprise, was near the 15,000-barrel limit of the amount of oil it could process, and because of concern that closing more vents would create more pressure that would force the cap off.

Officials say it is not yet possible to gauge what fraction of the total flow is being captured and what fraction is still escaping.

A second system is being prepared that would use the pipes and other equipment installed for the failed “top kill” effort two weeks ago to collect oil from the well. That oil would be siphoned up to a rig, the Q4000, that is at the site. The Q4000 is not equipped to separate gas and oil, however, so crews are hurriedly modifying it to do so.

BP officials have said the system using the Q4000 could be ready by next weekend.

At a technical briefing in Houston on Monday, Kent Wells, a senior BP executive, said that next month engineers would replace the cap with alarger device that will make a better seal. The device, which is currently being designed, would be a "sealing valve," he said. It would be connected to a new, floating riser pipe and subsea buoy that would allow collection ships to disconnect and disconnect faster in the event of a hurricane.