Royal Dutch Shell says slick is not from its nearby rigs.
The U.S. Coast Guard and oil company experts on Thursday used aircraft and undersea probes to monitor and search for the source of a 10-mile long oil sheen in the Gulf of Mexico.
The one-mile-wide slick is located some 130 miles southeast of New Orleans, Coast Guard spokeswoman Elizabeth Bordelon told AFP.
"At first light this morning Air Station New Orleans sent out a ... helicopter with a pollution investigator aboard to do an overflight," Bordelon said.
The sheen is between two offshore oil rigs owned by oil giant Royal Dutch Shell (IW 1000/2), Bordelon said.
However, Shell said the oil -- which it estimated at six barrels -- did not come from its rigs.
Workers with the Bureau of Safety and Environmental Enforcement, a US office that enforces rules for offshore oil rigs, first noticed the sheen at 12:30 p.m. local time Wednesday.
Bureau personnel, "while offshore, spotted an oil sheen" and immediately notified Shell, the office said Thursday.
Shell reported the sheen to the Coast Guard National Response Center more than four hours later, at 4:45 p.m., according to a Coast Guard statement.
The BSEE said it "directed Shell to conduct a seafloor assessment using a remote operated vehicle," and also identified area pipelines and told pipeline operators "to begin survey of their lines."
Shell said it conducted a thorough inspection of its assets but found "no sign of leaks."
"We are confident at this time that the sheen did not originate from Shell operations," Shell said, adding that "out of prudent caution" it will continue to "respond to the sheen."
The company said it activated the Louisiana Responder, ship designed to skim oil and contain it with booms, and deployed "two remote operating vehicles to inspect Shell and non-Shell infrastructure and search for potential naturally-occurring seeps in the area."
Shell's share price slid around 5% on news of the sheen discovery in London trading, but recovered to end Thursday's session down 0.75% at 2,051.85 pence on London's FTSE 100 index, which closed up 1.34%.
In U.S. trading, Shell shares opened down at $66.61, but rebounded to around $68.07 by midday.
"We are treating this very seriously, as we do all reports of possible pollution," said Coast Guard Captain Jonathan Burton.
Together with state and local officials, "we will ensure that all measures are taken to fully investigate and, if necessary, mitigate any impact this could potentially have," said Burton.
The source of the sheen "and the responsible party are unknown at this time," the statement added.
The Coast Guard-operated National Response Center is the federal government point of contact where oil and chemical spills in the United States are reported.
The Gulf of Mexico region is still recovering from the disastrous 2010 BP (IW 1000/3) oil spill. The spill blackened beaches in five states and devastated the Gulf Coast's tourism and fishing industries.
It took 87 days to cap BP's runaway well 5,000 feet below the water surface as it spewed 4.9 million barrels (206 million gallons) of oil into the Gulf of Mexico.
Copyright Agence France-Presse, 2012