The first oil from the Gulf of Mexico spill has entered an ocean current that could take it to Florida and up the east coast of the US, scientists say.
The European Space Agency said satellite images suggested oil could reach the coral reefs of the Florida Keys within six days.
"We have visible proof that at least oil from the surface... has reached the current," said Dr Bertrand Chapron.
The ESA images show a stream of oil extending south into the Loop Current.
Meanwhile, the US Coast Guard said tests showed that tar balls that washed up on Florida beaches in recent days did not originate from the oil spill off Louisiana.
It is unclear where the tar balls came from, coast guard officials said.
Turbulent system
ESA scientists described the Loop Current as a "conveyor belt" that joins the Gulf Stream, the most important current in the northern hemisphere.
If oil is dragged into the Gulf Stream, it could be carried up the east coast of the US, they said.
The scientists warned that the turbulent Loop Current could mix the oil and water, making it difficult to track the oil's progress in the coming days.
"This might remove the oil film on the surface and prevent us from tracking it with satellites, but the pollution is likely to affect the coral reef marine ecosystem," Dr Fabrice Collard said.
