The containment capping stack is seen in this image captured from a BP live video feed from the Gulf of Mexico on Tuesday. (BP/Reuters)

U.S. President Barack Obama says the struggle to stop the flow of oil from a blown-out well into the Gulf of Mexico is finally nearing an end.

BP said in a statement Wednesday that it has successfully pumped heavy drilling mud into its well in the Gulf to create a "static condition," a major milestone in the company's efforts to permanently plug the well.

"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 Wednesday.

The company started pumping mud into the well Tuesday, stopping after roughly eight hours.

The pressure in the well dropped quickly in the first 90 minutes of the static kill procedure, a sign that everything was going as planned, well site leader Bobby Bolton said.

"The well pressure is now being controlled by the hydrostatic pressure of the drilling mud, which is the desired outcome of the static kill procedure," BP said in a statement Wednesday.

'Static kill' optimism

BP spokeswoman Sheila Williams called the procedure a "milestone" and said it's a step toward killing the well, which started leaking after a deadly blast tore through an offshore drilling rig on April 20.

Teams are now monitoring the well to ensure it remains stable, BP said, and workers may or may not be required to inject more mud into the well at some point.

Obama said that the static kill appears to be working — but he reiterated that the cleanup efforts are far from over.

He said recovery efforts will continue, and emphasized the importance of reversing the damage caused by the spill.

Obama said the government must continue to hold polluters accountable for "the destruction they caused" and said the government will stand by Gulf residents for "however long it takes, until they are back on their feet."

Federal officials won't declare victory until they pump in mud and then cement from the bottom of the well, and that won't happen for several weeks.

BP said in a statement that a relief well remains "the ultimate solution to kill and permanently seal the well."

Oil in Gulf dissipating: report

Meanwhile, White House energy adviser Carol Browner said that a new assessment found that about 75 per cent of the oil has been captured, burned off, evaporated or broken down in the Gulf.

"It was captured. It was skimmed. It was burned. It was contained. Mother Nature did her part," Browner told NBC's Today show. On ABC's Good Morning America, she said about 25 per cent of the oil remained.

The estimates — from a report by the National Incident Command — say about a quarter of the oil that spilled was removed by recovery, burning and skimming. Another eight per cent was dispersed by chemical means. About a quarter of the oil has evaporated or dissolved and 16 per cent has been naturally dispersed.

Obama also addressed the report, saying the results indicated that the "vast majority of the spilled oil has been dispersed from the water."

Scientific teams recently estimated that the damaged well released roughly 4.9 million barrels of oil, although not all of the oil flowed into the Gulf.

"Containment activities conducted by BP under U.S. direction captured approximately 800,000 barrels of oil prior to the capping of the well," U.S. spill response officials said in a statement.

BP placed a temporary containment cap on the well on July 15, temporarily stemming the flow of oil into the Gulf.

With files from The Associated Press