The first heavy oil from a giant Gulf of Mexico spill sloshed ashore in fragile Louisiana marshlands on Wednesday and part of the mess entered a powerful current that could carry it to Florida and beyond.

The developments underscored the gravity of the situation as British energy giant BP raced to capture more crude gushing from a ruptured well a mile (1.6 km) beneath the surface. The spill is threatening an ecological and economic disaster along the US Gulf Coast and beyond.

"The day that we have all been fearing is upon us today," Louisiana Governor Bobby Jindal said after a boat tour to the southernmost point of the Mississippi River estuary.

"This wasn't tar balls. This wasn't sheen. This is heavy oil in our wetlands," he told a news conference. "It's already here but we know more is coming."

Officials had previously reported debris in the form of tar balls, or light surface "sheen" coming ashore in outlying parts of Louisiana, Mississippi and Alabama.

The marshes are the nurseries for shrimp, oysters, crabs and fish that make Louisiana the leading producer of commercial seafood in the continental United States and a top destination for recreational anglers. The United States has already imposed a large no-fishing zone in waters in the Gulf seen affected by the spill.

Meanwhile, the US government's top weather forecaster said a small portion of light sheen from the giant oil slick had entered the Loop Current, which could carry the oil down to the Florida Keys, Cuba and even up the US East Coast.

BP, its reputation on the line in an environmental disaster that could eclipse the 1989 Exxon Valdez spill in Alaska, has marked some progress at siphoning some of the oil from the well, which ruptured after an April 20 explosion of the Deepwater Horizon rig that killed 11 workers.

BP said it is now siphoning about 3,000 barrels (126,000 gallons/477,000 liters) a day of oil, out of what the company estimated was a 5,000 barrels (210,000 gallons/795,000 liters) a day gusher. The company could begin injecting mud into the well as early as Sunday in a bid to permanently plug the leak.

'Not rocket science'

A US congressional panel heard testimony from experts who said the spill rate could be many-fold larger.

"This is not rocket science," said Steve Wereley, associate mechanical engineering professor at Purdue University, who pegged the spill's volume at about 70,000 barrels per day. "All outside estimates are considerably higher than BP's."

BP Chief Operating Officer Doug Suttles said on Wednesday its 5,000-barrels-a-day estimate was "highly" uncertain.

BP shares closed down nearly 2 per cent in London on Wednesday, extending recent steep losses.

Political fall-out also continues. The US Interior Department said on Wednesday its embattled Minerals Management Service will be broken up into three separate divisions, as part of an effort to restructure the way the department handles offshore energy production.

Top Democrats in the US Senate urged President Barack Obama to order immediate, enhanced inspections of all offshore oil rigs and production platforms.

And another company entered the fray. Schlumberger, the world's largest oilfield services company, said it had a crew on the Deepwater Horizon that departed only hours before the explosion and fire that engulfed the rig.

The company, which had not previously revealed its work on the Horizon, said in an e-mailed statement it performed wireline services for BP on the rig in March and April, completing the last services on April 15 and leaving a crew on standby in case any more were needed.

Florida's tourism gained a respite when tar balls found on Keys beaches were shown not to come from the Gulf of Mexico oil leak, but officials said the $60 billion-a-year industry was already taking a beating from the month-old spill.

The Coast Guard said laboratory tests had shown that 50 tar balls found this week on the Lower Keys -- a mecca for divers, fishermen and beach goers -- were not from the Gulf spill.

'Not out of the woods'

Local tourism authorities said damage had already been inflicted by the negative publicity linked to the spill.

"Even if we don't get even a gumball-sized tar ball down here in the next month, there has already been significant perception damage to Florida Keys and Florida tourism," said Andy Newman of the Monroe Tourism Development Council.

"We understand we are not out of the woods yet, that there's more oil out there," he said.

Tar balls have also been found on the Texas coast and were being tested, but a Coast Guard official said it was "highly unlikely those tar balls in Texas are related to this spill.”

A Louisiana agency said a Kemp's Ridley sea turtle, an endangered species in the state, had been brought in and cleaned of oil after biologists discovered it off the coast.

The spill has also prompted rare talks between US and Cuban officials in Havana, with forecasters predicting that oil could reach Cuban shores.

Wildlife and environmental groups accused BP of holding back information on the real size and impact of the growing slick, and urged Obama to order a more direct federal government role in the spill response.

In prepared testimony for a congressional committee, National Wildlife Federation President Larry Schweiger said BP had failed to disclose results from its tests of chemical dispersants used on the spill. He also said it had tried to withhold video showing the true magnitude of the leak.

"The federal government should immediately take over all environmental monitoring, testing and public safety protection from BP," he said. "The Gulf of Mexico is a crime scene and the perpetrator cannot be left in charge of assessing the damage."

The spill has forced Obama to put a hold on plans to expand offshore oil drilling and has raised concerns about planned oil operations in other areas like the Arctic. – Reuters