Officials are aiming to clean up all the oil from the BP spill from Florida and Alabama beaches before next year's spring tourist season.
The beaches are a popular destination for university students and other visitors during spring break, which peaks around March.
That's a "very aggressive timetable" and some beaches may not meet that goal, U.S. Coast Guard Rear Admiral Paul Zukunft, the federal point man for the disaster, said in New Orleans.
He said Wednesday there's no set schedule for cleaning the remaining 945 kilometres of oiled shoreline. Scattered areas of heavy oil total about 42 kilometres, Zukunft said.
He said heavy north winds and beach erosion in Florida exposed tar mats, adding a couple of kilometres to the heavily oiled total from last week.
"Some of our more persistent oil is in that sand column on both recreational beaches and national park shorelines," he added.
It may be impossible to remove all the oil from some areas, especially Louisiana's delicate coastal marshes, he said.
"If we reach a point where any further cleanup has no net environmental benefit, we will terminate that phase of the response."
About 135 shrimp and fishing boats are still aiding in Gulf of Mexico oil spill cleanup duties.
The April 20 explosion killed 11 workers and led to the largest offshore oil spill in U.S. history.