Part of the oil slick was only 16 miles offshore and closing in on the Mississippi River Delta, the marshlands at the southeastern tip of Louisiana where the river empties into the ocean. Already 100,000 feet of protective booms have been laid down to protect the shoreline, with 500,000 feet more standing by, said Charlie Henry, an oil spill expert for the National Oceanic and Atmospheric Administration, at an earlier news conference on Wednesday.

On Wednesday evening, cleanup crews began conducting what is called an in-situ burn, a process that consists of corralling concentrated parts of the spill in a 500-foot-long fireproof boom, moving it to another location and burning it. It has been tested effectively on other spills, but weather and ecological concerns can complicate the procedure.

Such burning also works only when oil is corralled to a certain thickness. Burns may not be effective for most of this spill, of which 97 percent is estimated to be an oil-water mixture.

A burn scheduled for 11 a.m. Wednesday was delayed. At 4:45 p.m., the first small portion of the spill was ignited. Officials determined it to be successful.

Walter Chapman, director of the Energy and Environmental Systems Institute at Rice University, said a 50 percent burn-off for oil within the booms would be considered a success. Admiral Landry called the burn “one tool in a tool kit” to tackle the spill. Other tactics include: using remote-controlled vehicles to shut off the well at its source on the sea floor, an operation that has so far been unsuccessful; dropping domes over the leaks at the sea floor and routing the oil to the surface to be collected, an operation untested at such depths that would take at least two to four more weeks; and drilling relief wells to stop up the gushing cavity with concrete, mud or other heavy liquid, a solution that is months away.

The array of strategies underscores the unusual nature of the leak. Pipelines have ruptured and tankers have leaked, but a well 5,000 feet below the water’s surface poses new challenges, officials said.

Reached in southern Louisiana on Wednesday, where he was visiting the response team’s command center, Tony Hayward, the chief executive of BP, said he did not yet know what went wrong with the oil rig. BP, which was leasing the rig from Transocean, is responsible for the cleanup under federal law.