NEW ORLEANS — BP’s risky “top kill” of its ruptured Deepwater Horizon well appeared to stop the oil flow Thursday, even as new data showed the Gulf of Mexico leak had already become the worst in U.S. history.

Moving to ensure the United States is never hit with such a disaster again, President Barack Obama was clamping down on the oil industry, putting on hold any deepwater oil exploration or drilling for the next six months.

Coast Guard chief Thad Allen, who is coordinating the federal government’s battle against the spill, said the “top kill” maneuver begun on Wednesday by BP to plug the leak had stopped the gush of oil from the mile-deep well.

But he cautioned it was still too early to declare victory as the British energy giant pumps heavy drilling liquids, dubbed “mud,” into the fractured wellhead to beat back the flow of oil, before sealing it with concrete.

“They’ve been able to stabilize the wellhead, they’re pumping mud down it. They’ve stopped the hydrocarbons from coming up,” Allen told local radio WWL First News. But he cautioned that while they were optimistic, “there is no reason to declare victory yet. We need to watch it very, very carefully.”

BP officials said they hoped to know later Thursday if the operation had succeeded.

It was the first flicker of good news for BP in the five weeks since the Deepwater Horizon rig leased from Transocean exploded on April 20, killing 11 workers, and then sank.

But government scientists confirmed fears that the Gulf of Mexico spill is set to be the worst in U.S. history, saying oil was flowing out at a rate up to four times higher than previously estimated.

Unveiling new data, they said the oil had been gushing from the burst pipe at a rate of between 12,000 to 19,000 barrels a day — much higher than the previous estimate of 5,000 barrels a day.

Under such a scenario, that would mean that between 18.6 million and 29.5 million gallons of oil have poured into the Gulf.

The 1989 Exxon Valdez disaster off Alaska saw some 11 million gallons of crude spilled into the state’s pristine waters, in what was until now the worst U.S. oil disaster.

BP has come under increasing pressure from the Obama administration and furious residents of the Gulf region helplessly watching oil wash ashore, with 100 miles of Louisiana’s fragile coastal wetlands and beaches now contaminated. The sticky crude is already imperiling rare species of animal and plant life, and has also led to a major fishing ban in the marine rich waters.

In a new development highlighting environmental and health problems, all 125 commercial fishing boats helping to clean up the oil off Louisiana’s Breton Sound were recalled after four workers reported health problems. The crew members aboard three separate vessels working in the area reported experiencing nausea, dizziness, headaches, and chest pains, officials said. It raised new questions about the risks of working with the thick gobs of oil washing up on shores here and the toxicity of tens of thousands of gallons of chemical dispersants used by BP to break up the slick.

Amid the desperate cleanup, there was more ominous news from U.S. experts, who warned the upcoming hurricane season could be one of the worst on record. It is feared that high winds could sweep huge oil-soaked waves further up onto the shores of Louisiana and its neighbors, Alabama, Mississippi, and Florida.

A White House aide said Obama would extend for six months a moratorium on offshore oil drilling in deep water. He also will delay planned oil exploration projects off Alaska, and cancel plans to lease drilling rights in the western Gulf of Mexico and off the coast of Virginia, officials said.

Meanwhile amid a litany of reported failures at the well, The New York Times said BP chose a casing for the deepwater well that was the riskier of two options, partly because it made “the best economic case.” Citing a BP document, the Times said it was feared that if the cement around the casing pipe chosen by BP did not seal properly, gases could leak all the way to the wellhead, where only a single seal would serve as a barrier. The other option under consideration would have provided two barriers in case of a gas leak, the Times said.