Tests have shown that underwater oil plumes have travelled at least 40 miles from a leaking well in the Gulf of Mexico, the US government says.
Scientists noted that concentrations of oil in the plumes were "very low."
Meanwhile, US Coast Guard chief Thad Allen has said a containment cap placed on a blown-out well in the Gulf of Mexico last week is helping to contain more of the leaking oil.
President Barack Obama has criticised BP's chief executive over the disaster.
Mr Obama told US network NBC that he would have fired Tony Hayward over remarks the British chief executive made on behalf of BP.
Jane Lubchenco, the head of the US National Oceanic and Atmospheric Administration (NOAA), said water samples had been taken at three sites by a University of South Florida research vessel.
Surface samples confirmed the presence of low concentrations (five parts per million) of oil 40 miles (64km) away from the BP site.
Ms Lubchenco told the Associated Press (AP) news agency that officials had finished "fingerprinting" the oil in order to confirm that it came from the BP spill.
"We've been tracking where the oil is going at the surface and where it is going below the surface," said Ms Lubchenco said.
"The BP oil spill is a human tragedy and an environmental disaster."
NOAA's testing backs up reports from scientists at a number of universities who had suggested that plumes of oil were suspended beneath the surface of the Gulf.
Scientists warn that oil plumes are very difficult to clean up, and they could damage the Gulf's abundant sea life by depleting oxygen in the water.
BP said 10 days ago that its own testing had found "no evidence" of large plumes of oil underwater.
"The oil is on the surface," Mr Hayward is quoted by AP as saying on 30 May. "There aren't any plumes."
Adm Allen said in a press conference on Tuesday that BP had contained more than 14,800 barrels of oil from the leaking wellhead in the past 24 hours.
"We continue to optimise production, make sure we can take as much oil out of that stream as we can right now," said Adm Allen.
Oil from the Deepwater Horizon spill has been found on beaches and wetlands along more than 100 miles (161km) of coast from Louisiana to the Florida Panhandle.
A containment cap was lowered over the well last week to siphon off the oil to a containment ship.
Oil has been leaking into the Gulf of Mexico since the Deepwater Horizon rig exploded and sank off the coast of the US state of Louisiana on 20 April.