HOUSTON -- Royal Dutch Shell PLC asserted yesterday that a long slick sighted in the Gulf of Mexico did not come from any of its operations, though the company and government are still investigating.

Shell alerted the U.S. Coast Guard's National Response Center late Wednesday that its employees discovered a "sheen" about 1 mile wide and 10 miles long, floating on the surface in waters between two Shell-operated platforms. Exxon Mobil Corp., ConocoPhillips and BP PLC also hold partial interests in the operations, named Mars and Ursa.

Late yesterday, Shell said that observations by it, the U.S. Coast Guard and the Bureau of Safety and Environmental Enforcement determined that light, thin material of unknown origin had broken apart and appeared to be dissipating.

Earlier, Shell said crews aboard the production platforms reported no problems. The company says it's confident that the oil sheen isn't emanating from those operations, but nevertheless the company is taking extra precautions in light of the fact that the incident comes almost exactly two years since the onset of BP's Macondo well blowout and record Gulf oil spill.

The Coast Guard sent out a team to monitor the sheen, with its own oil spill experts tasked to determine the source. Meanwhile, Shell says it has sent a spill-response vessel to the site, in waters roughly 130 miles southeast of New Orleans.

The vessel, Louisiana Responder, is equipped with "skimming and boom capabilities," said Shell spokeswoman Kelly op de Weegh in an email. She added that so far the size of the sheen, though spread over a large area, was estimated to be of minimal volume, roughly the equivalent of six barrels of oil as of the company's latest report on the incident.

"Although we are confident at this time that the sheen did not originate from Shell operations, out of prudent caution we will continue to respond to the sheen," the company said on a notice posted on its website.

Additional response measures include aerial surveillance to watch the sheen and try to determine its origin, and two underwater remote-operated vehicles (ROVs) sent to assess the conditions of equipment in the region, including at production zones not operated by Shell. Shell said the ROVs are still being used to investigate the incident, searching for flaws in man-made equipment but also looking for any natural seeps that could have caused it. The Gulf of Mexico is known to leak naturally about 1 million barrels of oil a day, creating a "biosheen" that’s eaten away by bacteria.

The 10-mile-long sheen is very close to the site of the Macondo well disaster, toward the eastern end of the Central Planning Area in the Department of Interior's offshore drilling lease system.

The company's share prices tumbled in stock trading in London early yesterday as fears of another Gulf spill grew. But Shell's share price on the New York Stock Exchange recovered after news spread that the company was insisting its operations didn't cause the sheen.

The company said monitoring and investigation of the source of the sheen will continue tomorrow.