On August 4, the US government said clean-up efforts and natural degradation had elimated some 74 percent of the 4.9 million barrels of oil believed to have spilled from the ruptured Macondo well into the Gulf's seawater.

"The vast majority of the oil from the BP oil spill has either evaporated or been burned, skimmed, recovered from the wellhead or dispersed," the National Oceanic and Atmospheric Administration wrote.

But scientists from the University of Georgia believe the government analysis ignores the fact that much of the spilled oil remains in the sea, just in smaller droplets or broken-down forms and is based on faulty assumptions.

"We just reanalyzsed this report and then we went to the next step," said Charles Hopkinson, one of the five-member team who reexamined the estimates.

"We came up to 70 to 79 percent must still be out there," he told AFP.

"They said about eight percent was chemically dispersed, but there is no reason not to think it's not still out there. They say that 16 percent was naturally dispersed, and again, there's no reason to think that is not still out there," he said.

The team also challenged the government's estimates of how the oil would dissolve and evaporate, arguing their suggested rates were optimistic and unlikely.

"One major misconception is that oil that has dissolved into water is gone, and therefore harmless," Hopkinson, a marine scientist, added.

"The oil is still out there, and it will likely take years to completely degrade. We are still far from a complete understanding of what its impacts are."

A US government official who spoke to AFP on condition of anonymity on Tuesday acknowledged the figures announced August 4 may have been inaccurate, and current estimates suggested closer to 50 percent of the spilled oil had actually been eliminated from the Gulf's waters.

He said around 25 percent of the spilled oil was either recovered, burnt or skimmed, and another 25 percent was believed to have been dispersed, either naturally or with the use of chemical dispersants.

He stressed that the government remained concerned about the effects of any oil remaining in the Gulf, but said any difference between the official estimates and the researchers' analysis "is really one of math."

Hopkinson disputed that characterization, drawing attention to the different assumptions being used by the research team and the government.

The government estimates, for example, assume a significant amount of oil floated to the surface, where it was able to evaporate, but Hopkinson said the gradient of water density at the spill site made it unlikely the oil would have risen far from the well rupture on the seafloor.

Ed Overton, a marine scientist at Louisiana University who reviewed the government's estimates, said the report "was done to give people some idea."

"The NOAA report was done to help the response and the (University of) Georgia report was done to say 'Look there is potential damage and particularly in the offshore area,'" he told AFP.

"The numbers of both reports are not terribly far off," he added. "Both of them are estimates and estimates are a guess. It's a guess by educated folks, but it's still a guess."

Overton said he believed a full assessment of the amount of oil in the ocean could take at least two years.

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