HOUSTON (Reuters)—A giant oil slick from a deadly offshore drilling rig explosion could hit the fragile U.S. Gulf Coast shoreline this weekend, while the Coast Guard on Tuesday scrambled to staunch an underwater leak that's gushing 1,000 barrels a day into the Gulf of Mexico.

The leaking well, 5,000 feet under the ocean surface off Louisiana's coast, has created an oil sheen and emulsified crude slick with a circumference of about 600 miles, covering about 28,600 square miles, the Coast Guard said Tuesday. That's slightly bigger than the U.S. state of West Virginia.

Zug, Switzerland-based Transocean Ltd.'s Deepwater Horizon sank on April 22, two days after it exploded and caught fire while finishing a well for BP P.L.C. about 40 miles southeast of the mouth of the Mississippi River.

The spill was 36 miles off the coast of Louisiana on Monday afternoon, but a shift in winds could push the spill inland to the Louisiana coast by this weekend, according to forecasters at AccuWeather.

"The wind will nudge the oil slick more to the north-northwest," said Dan Kottlowski, senior meteorologist at AccuWeather. "It might make it onshore over the southeast Louisiana coast first," and later threaten beaches in Mississippi, Alabama and Florida, Mr. Kottlowski said.

The Coast Guard is using underwater robots to try to activate a cutoff valve on the ocean floor to stop the oil flow. BP may deploy devices that look like upside down bowls over the well within two to four weeks to capture escaping oil before it reaches the surface.

But London-based BP, which is financially responsible for the cleanup, will have to drill relief wells to plug the runaway well under the seabed, a process which could take months. The Coast Guard has staged thousands of feet of boom, or floating oil barriers, to keep the oil away from the most sensitive coastal areas.

"We're terrified about the likely impacts to the Gulf's threatened and endangered sea turtle species," said Aaron Viles, campaign director of the Gulf Restoration Network in New Orleans. Sperm whales, dolphins, porpoises and sea birds could also be fouled by the oil slick, Mr. Viles said.

Eleven workers from the rig are missing and presumed dead in what is the worst oil rig disaster in almost a decade.

The spill, however, is not comparable with the infamous Exxon Valdez disaster, which spilled about 11 million gallons of oil into the Prince William Sound in Alaska when it ran aground in 1989. BP's well is spewing about 42,000 gallons of oil a day into the ocean, the Coast Guard estimates.

But as the oil spill grows, so does the chance that it will impact efforts by the U.S. Congress and President Barack Obama to open more offshore areas to limited oil and gas drilling.

President Obama on March 31 called for a limited expansion of offshore oil and gas drilling in an effort to win Republican support for new proposals to fight climate change.

But President Obama's plan left the thorniest issues, such as how royalties that oil companies pay to drill on federal offshore waters will be shared with coastal states, for lawmakers to settle.

"A big spill could make those tough choices even tougher," said Kevin Book, managing director at ClearView Energy Partners L.L.C. in Washington. "It isn't likely to change the status quo, however."

A bipartisan group of senators had aimed to unveil a climate change bill on Monday that would have included some offshore drilling expansions. But the plan was called off when Sen. Lindsey Graham, R-S.C., raised eleventh-hour objections related to immigration.