For the US Coast Guard, the mighty Mississippi River is its "best friend" in the fight against the oil spill.

Trying to keep the spreading oil at bay, authorities have diverted the river into dozens of canals. But fishermen and ecologists are concerned an environmental disaster could be in the offing.

After crossing the United States from north to south, the 2,350-mile (3,800-kilometer) long river delivers its waters to the Gulf of Mexico through a sprawling delta formed by dozens of canals.

With the outflow measuring between 7,000 and 20,000 cubic meters of water per second, the Mississippi represents a powerful natural ally of US authorities scrambling to control what could become the worst oil spill in US history.

"The Mississippi River, it is our best friend," US Coast Guard Captain Edwin Stanton told AFP. "Because it's pushing and keeping the oil away from the delta."

The BP-leased Deepwater Horizon oil rig exploded and sank in the gulf last month, rupturing a riser pipe that has been spewing hundreds of thousands of gallons of crude into the sea each day.

The US Coast Guard told AFP that oil was washing ashore in at least two new locations -- Whiskey Island, Louisiana and Long Beach, Mississippi.

"We sent crews to assess what type of oil, and we determined it's 'soft patties' on Whiskey Island and 'tar balls' on Long Beach," said Petty Officer Erik Swanson.

Oil globs have also washed ashore on barrier islands in Alabama.

The appearance of oil in new locations highlights the urgency of efforts to contain the spill, which experts warn may be growing at a rate close to 70,000 barrels (2.9 million gallons) a day, more than 10 times faster than previous Coast Guard estimates.

The new findings suggest the spill has already eclipsed the 1989 Exxon Valdez spill, the worst environmental disaster in US history.

In the face of the catastrophe, US authorities have decided to open the locks at some of the canals to increase the outflow to maintain the oil slick away from the ecologically fragile delta.

But ecologists and fishermen worry this decision could backfire at a time when many fish and shellfish have entered their reproductive season.

"It pushes the juvenile shrimp right into the oil, and we are going to lose them all," complained Clint Guidry, president of the Louisiana Shrimpers Association.

He said the water coming out from the Mississippi was of low purity and untreated.

"Because of the flooding in the north, you have chemical products that are on the ground, in the communities, and go in the Mississippi," Guidry explained. "And I really don't agree with this decision."

Aron Viles, a campaign director for the Gulf Restoration Network also warned that the increased flow of Mississippi water was pushing shrimp right into the oil.

"We would like to have better answers before we proceed," he said. "Better answers about the impact on shrimp, about the impact on oysters, about the impact on the larvae."

Viles said his organization had asked the US Environmental Protection Agency (EPA) some of these questions but had not received any answers yet.

Paul Horsnan, an oil expert for Greenpeace, acknowledged that everybody was working in an unknown situation, but urged caution with any steps that are being taken.

"Whether it's injecting dispersants, whether it's spreading booms around, or diverting the Mississippi, everybody tries to see what to do to improve the situation," he said. "We must not do any oil spill response technics that cause further damages to the environment."