Oil from a massive spill in the Gulf of Mexico was starting to ooze ashore in the southeastern U.S., threatening birds and other wildlife along fragile islands and barrier marshes and putting one of the world’s richest seafood grounds in peril.

Crews in boats were patrolling coastal marshes early Friday looking for areas where the oil has flowed in, the U.S. Coast Guard said. Storms loomed that could push tide waters higher than normal through the weekend, the National Weather Service warned.

A top adviser to President Barack Obama said on Friday that no new oil drilling would be authorised until authorities learn what caused the explosion of the rig Deepwater Horizon. David Axelrod told ABC television that “no additional drilling has been authorized and none will until we find out what has happened here.” Mr. Obama recently lifted a drilling moratorium for many offshore areas, including the Atlantic and Gulf areas.

The leak from a blown-out well a mile underwater is five times bigger than first believed. Faint fingers of oily sheen were reaching the Mississippi River delta late Thursday, lapping the Louisiana shoreline in long, thin lines. Thicker oil was about 8 km offshore. Officials have said they would do everything to keep the Mississippi River — the largest U.S. river, running up the country’s middle — open to traffic.

Coast Guard Rear Adm. Sally Brice-O’Hara faced questions on all three network television morning shows on Friday about whether the government has done enough to push oil company BP PLC to plug the underwater leak and protect the coast. Rear Adm. Brice-O’Hara said the federal response led by the Coast Guard has been rapid, sustained and has adapted as the threat grew since a drill rig exploded and sank last week, causing the seafloor spill.

The oil slick could become the nation’s worst environmental disaster in decades, threatening to eclipse even the Exxon Valdez in scope. It imperils hundreds of species of fish, birds and other wildlife along the Gulf Coast, one of the world’s richest seafood grounds, teeming with shrimp, oysters and other marine life.

“It is of grave concern,” David Kennedy of the National Oceanic and Atmospheric Administration, told The Associated Press about the spill. “I am frightened. This is a very, very big thing. And the efforts that are going to be required to do anything about it, especially if it continues on, are just mind-boggling.”

Oil clumps seabirds’ feathers, leaving them without insulation — and when they preen, they swallow it. Prolonged contact with the skin can cause burns, said Nils Warnock, a spill recovery supervisor with the California Oiled Wildlife Care Network at the University of California-Davis. Oil swallowed by animals can cause anaemia, haemorrhaging and other problems, said Jay Holcomb, executive director of the International Bird Rescue Research Centre in California.

The spewing oil — about 795,000 litre a day — comes from a well drilled by the rig Deepwater Horizon, which exploded in flames on April 20 and sank two days later. BP was operating the rig that was owned by Transocean Ltd. The Coast Guard is working with BP to deploy floating booms, skimmers and chemical dispersants, and set controlled fires to burn the oil off the water’s surface.

Protective boom has been set out on Breton Island, where colonial species such as pelicans, gulls and skimmers nest, and at the sandy tips of the passes from the Mississippi River’s birdfoot delta, said Robert Love, a State wildlife official.

The leak from the ocean floor proved to be far bigger than initially reported, contributing to a growing sense among some in Louisiana that the government failed them again, just as it did during Hurricane Katrina in 2005. President Barack Obama dispatched Cabinet officials to deal with the crisis.

Cade Thomas, a fishing guide in Venice, worried that his livelihood will be destroyed. He said he did not know whether to blame the Coast Guard, the government or BP.

“They lied to us. They came out and said it was leaking 1,000 barrels when I think they knew it was more. And they weren’t proactive,” he said. “As soon as it blew up, they should have started wrapping it with booms.”