The US 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 1.6 kilometer (one mile) wide slick is located some 210 kilometers (130 miles) south-east 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, 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 1730 GMT Wednesday.
Bureau personnel, "while offshore, spotted an oil sheen in the central Gulf of Mexico" and immediately notified Shell, the office said in a statement Thursday.
The bureau said it "directed Shell to conduct a seafloor assessment using a remote operated vehicle," and has also identified pipelines in the area and "directed the pipeline operators to begin survey of their lines."
Shell said it conducted a thorough inspection of its assets and found "no sign of leaks," the company said in a statement.
"We have also confirmed there are no well control issues associated with our drilling operations in the area."
"We are confident at this time that the sheen did not originate from Shell operations," Shell said, but added that "out of prudent caution" 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.
Shell also said it has deployed "two remote operating vehicles to inspect Shell and non-Shell infrastructure and search for potential naturally-occurring seeps in the area."
Shell earlier said it notified the US National Response Center "of a light sheen" in the central Gulf of Mexico.
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 oil spill. The spill blackened beaches in five US states and devastated the Gulf Coast's tourism and fishing industries.
It took 87 days to cap BP's runaway well 5,000 feet (1,500 meters) below the water surface as it spewed 4.9 million barrels (206 million gallons) of oil into the Gulf of Mexico.
Explore further: Muddy forests, shorter winters present challenges for loggers