Mr. Fryar said that for a second day, crews were injecting chemical dispersant into the oil as it flowed from the main leak. Dispersant, which is more conventionally used on the water surface, breaks the oil into small droplets and reduces its buoyancy, so it will sink to the bottom.

Mr. Fryar said technicians were trying to determine whether it would be possible to inject the dispersant directly into the riser deep under the water so that it would mix better with the flowing oil. “We think this dispersant is highly effective,” he said. “We’re hoping the oil won’t make it to the surface.”

The impact of chemical dispersants on deepwater ecology is unclear.

Closer to land, crews continued to put out booms to stop the oil and were training volunteers in Mississippi and Florida to help minimize the impact if the slick reaches the beach, said John Curry, head of external affairs at BP.

Speaking to reporters in the rain in Venice, La., after talking with response teams that have amassed on the coast to try to stem the tide of oil lapping the shoreline, Mr. Obama vowed that the government would keep up the pressure on BP.

While he did not criticize the company in his public remarks, Mr. Obama’s comments reflected increased frustration in the administration with BP’s inability to plug the oil leak. The president again reiterated that American taxpayers would not foot the bill.

“BP is responsible for this leak — BP will be paying the bill,” Mr. Obama said.

Mr. Obama had initially not planned to visit the region until later this week at the earliest, White House officials said Friday afternoon. But by late Friday night, with criticism mounting that the government’s response was too slow, White House officials decided that the president needed to make the trip to the gulf on Sunday.

Image President Obama spoke to reporters in Victoria, La., on Sunday. Credit... Doug Mills/The New York Times

White House officials sent two Cabinet officials to appear on the Sunday television talk shows with the message that the administration was doing everything it could to take control of the spill and that it had been involved from the beginning. Homeland Security Secretary Janet Napolitano said on Fox News Sunday that the government had an “all hands on deck” approach to the disaster.