Gas is burned off on a ship collecting oil from a mile-deep oil leak in the Gulf of Mexico on Monday, while a relief well to relieve pressure on the leak is seen at the upper left. (Charlie Riedel/Associated Press)

U.S. government officials have expanded the boundaries of a fishing ban in the Gulf of Mexico, a precautionary move meant to ensure fish caught there have not been affected by the massive oil spill.

Fishing had already been shut down from the Mississippi River to the Florida Panhandle, an area representing about seven per cent of federal waters.

The expansion will close an additional 12 per cent of federal waters, for a total of 120,000 square kilometers, the National Oceanic and Atmospheric Administration announced Tuesday.

Jane Lubchenco of the National Oceanic and Atmospheric Administration said Tuesday her organization is monitoring the flow of the oil spill into a current that could take it to Florida. (Patrick Semansky/Associated Press)NOAA scientists are also surveying the Gulf of Mexico to determine if oil from the massive spill off Louisiana has entered a powerful current that could take it to Florida, administrator Jane Lubchenco told reporters.

She said aerial surveys show some tendrils of light oil close to or already in the loop current, which circulates in the Gulf and takes water south to the Florida Keys and the Gulf Stream.

Most of the oil is still dozens of kilometres away from the current.

Lubchenco said it will take about eight to 10 days after oil enters the current before it begins to reach Florida.

But researchers from the University of South Florida believe oil from the spill could reach Key West by Sunday if the huge plumes of crude already released get caught in the current.

The loop current is a ribbon of warm water that begins in the Gulf of Mexico and wraps around Florida. Some scientists project the current will draw the crude through the Keys and then up Florida's Atlantic Coast, where the oil might avoid the beaches of Miami and Fort Lauderdale but could wash up around Palm Beach.

Cost of response grows

As the oil spill grows, so too does the price tag for containing and cleaning it up, according to new numbers released by BP Tuesday.

The London-based oil giant has spent $625 million US on spill response, containment, relief well drilling, grants to Gulf states, settlements and federal costs, BP said on its website.

That is $175 million more than it had spent as of Thursday, outpacing estimates that the efforts would cost the company $10 million a day.

The latest tab includes grants announced Monday — worth $70 million — to promote tourism in Florida, Alabama, Louisiana and Mississippi.

Florida will receive $25 million; the other states are to receive $15 million each.

"The Gulf Coast is our home, too," said BP CEO Tony Hayward. "We are doing everything we can to plug the leak, contain the spill offshore and protect the shoreline."

Tar balls found

The grants and Hayward's comments came the same day U.S. Coast Guard officials responded to a report that 20 tar balls had washed up on a beach in Key West, Fla.

Three of 20 tar balls retrieved Monday by park rangers in Fort Zachary State Park in Key West, Fla. (U.S. Coast Guard/Associated Press)Park rangers found the balls — which were between seven and 20 centimetres in diameter — on a routine search of shoreline in Fort Zachary Taylor State Park.

The coast guard has shipped them to a laboratory for analysis to determine if they came from the BP spill or elsewhere. Tar balls can occur naturally or come from other sources such as ships.

Similar pollution or worse could endanger Florida's shoreline mangroves, seagrass beds and the third-longest barrier reef in the world, the 356-kilometre-long Florida Keys National Marine Sanctuary, which helps draw millions of snorkellers, fishermen and other tourists whose dollars are vital to the state's economy.

Pollutants can smother and kill corals — living creatures that excrete a hard exterior skeleton — or can hinder their ability to reproduce and grow. That, in turn, could harm thousands of species of exotic and colourful fish and other marine life that live in and around reefs.

With files from CBC News