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.
A "small portion" of oil sheen is in the Loop Current, which circulates in the Gulf, the National Oceanic and Atmospheric Administration (NOAA) said.
Diluted oil could appear in isolated parts of Florida if persistent winds pushed the current that way, it added.
European scientists warn the spill could reach Florida within six days.
Oil has been spewing into the Gulf since the offshore drilling rig Deepwater Horizon, leased by oil giant BP, exploded off the coast of Louisiana on 20 April and sank two days later.
Satellite images released by the European Space Agency (ESA) depict a streak of oil stretching south from the main slick into the Loop Current - a body of fast-flowing water coming from the Caribbean which the agency says is likely to propel oil towards Florida within six days.
Scientists at the NOAA, the US government's own climate body, broadly share that analysis, but say the oil is a "light sheen" representing just a small portion of the overall amount.
The oil would be "highly weathered" if it reached Florida, or could evaporate en route, it 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.
Meanwhile, the US Coast Guard said tests showed that tar balls that washed up on Florida beaches in recent days had not originated from the oil spill off Louisiana.
It is unclear where the tar balls came from, Coast Guard officials said.
Also on Wednesday, the US said it was having talks with Cuba over the spill.
Observers say the rare talks demonstrate a concern that the oil may be carried by currents far from the site of the Deepwater Horizon disaster.
In Louisiana, a lawyer has asked a panel of federal judges to consolidate more than 100 cases related to the oil spill into a single action.
Daniel Becnel asked that the growing number of cases against oil companies BP, Transocean, Halliburton and Cameron be combined and heard in Louisiana, the Associated Press news agency reported.
The lawsuits have been filed by commercial fishermen, restaurants, hotels and property owners and others who say the oil spill has cost them income.
A BP executive said this week that the company had paid out $15m (£10.4m) in claims, much of it to shrimpers and commercial fishermen who have little or no income because of the spill.
Meanwhile, astronauts and cosmonauts aboard the International Space Station have said they could see the oil spill while passing over the Gulf of Mexico.
"It looks very scary," Russian cosmonaut Oleg Kotov told reporters via a video link.