Heavy oil from the Gulf of Mexico spill threatened Louisiana marshlands on Thursday after washing ashore for the first time since a BP-operated rig exploded a month ago, sparking ecological disaster.

Calling it a "day that we have all been fearing," Louisiana Governor Bobby Jindal said on Wednesday that heavy oil - not simply tar balls or sheen - had entered the state's prized wetlands. "It's already here but we know more is coming," he said.

The marshes are the nurseries for shrimp, oysters, crabs and fish that make Louisiana the leading producer of commercial seafood in the continental U.S. A large no-fishing zone in Gulf waters seen as affected by the spill has been imposed.

Energy giant BP scrambled to contain crude from the gushing undersea well, which ruptured after an April 20 explosion of the Deepwater Horizon rig that killed 11 workers. The company said it is now siphoning about 3,000 barrels (126,000 gallons/477,000 liters) a day of oil, from what it has estimated was a 5,000 barrels (210,000 gallons/795,000 liters) a day gusher. The company said it could begin injecting mud into the well as early as Sunday in a bid to permanently plug the leak.

BP shares closed down nearly 2% in London on Wednesday, extending recent steep losses. Adding another name to the group of companies connected to the doomed rig, Schlumberger said it had a crew on the Deepwater Horizon that departed only hours before the explosion and fire that engulfed it. The world's largest oilfield services company had not previously revealed its work on the Horizon.

Fall-Out Increases

The discovery of heavy oil in marshlands at the southern tip of Louisiana's peninsula showed that authorities lacked the capacity to track undersea oil effectively, marine conservation biologist Rick Steiner said. It also called into question a containment effort that focused on oil on the surface of the Gulf, Steiner said.

"I am very confident that a lot of the oil that has come out has not surfaced yet and the government can't track subsurface plumes," said Steiner, a retired professor at the University of Alaska who has just spent a week on the Gulf coast.

Meanwhile, the U.S. government's top weather forecaster said a small portion of light sheen from the giant oil slick had entered the powerful ocean flow known as the Loop Current, which could carry the oil down to the Florida Keys, Cuba and up the U.S. East Coast. Wildlife and environmental groups accused BP of holding back information on the real size and impact of the growing slick, and urged President Barack Obama to order a more direct federal government role in the spill response. Obama plans to create a commission to investigate the cause of the spill, evaluate industry practices and study government oversight.

The U.S. 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 U.S. Senate urged Obama to order immediate, enhanced inspections of all offshore oil rigs and production platforms.

"Until we can ensure the safety of our offshore platforms, our nation's coastlines will be threatened by the possibility of more man-made catastrophes," the letter said.