A procedure to pump heavy mud into the blown-out BP oil well in the Gulf of Mexico has succeeded in plugging it, officials said Wednesday, and government scientists reported that most of the crude that gushed from it for three months has dissipated or been removed from the water.

President Obama cautioned that much recovery work remains, and other officials warned that the threat from the damaged Macondo well will not be eliminated until it is "killed" later this month with a relief well. Moreover, federal officials said, the long-term effects of the oil spill, the worst in the nation's history, remain to be determined.

But in view of a series of early setbacks in efforts to control the blown-out well, BP's announcement early Wednesday that it had reached a "static condition" came as a huge relief. The term meant that pressure inside the well was brought under control through a mud-pumping process that began Tuesday afternoon.

(Read an analysis here of whether the well is really dead.)

BP called the achievement "a significant milestone" and said it stopped pumping mud into the Macondo well after about eight hours because the effort had been successful.

"The well is now being monitored, per the agreed procedure, to ensure it remains static," the company said in a statement. "Further pumping of mud may or may not be required depending on results observed during monitoring."

In a speech in Washington, Obama called the development welcome news. He also pointed to the report released Wednesday by government scientists who found that an estimated 74 percent of the oil has been recovered, burned, dispersed, evaporated, consumed by microbes or otherwise removed from the gulf.

"The long battle to stop the leak and contain the oil is finally close to coming to an end, and we are very pleased with that," Obama said at a meeting of the AFL-CIO Executive Council.

Obama added: "Our recovery efforts, though, will continue. We have to reverse the damage that's been done. We will continue to work to hold polluters accountable for the destruction they've caused. We've got to make sure that folks who were harmed are reimbursed, and we're going to stand by the people of the region for however long it takes until they're back on their feet."

As they huddled in BP's operations center, federal officials tried to manage expectations, saying that even if the operation known as "static kill" went as hoped, the well at the bottom of the Gulf of Mexico would not be finally plugged until it was intercepted and cemented by a relief well that crews have been drilling for three months.

The federal official in charge of the oil spill response, retired Coast Guard Adm. Thad W. Allen, told a New Orleans television station Wednesday that pumping mud into the blown-out well took care of the immediate threat but that the "bottom kill" technique involving the 18,000-foot relief well would still go ahead.

"We've pretty much made this well not a threat, but we need to finish this from the bottom," Allen told WWL-TV.