The U.S. Coast Guard is checking reports of a 10-mile oil slick in the Gulf of Mexico near wells operated by Shell Oil.
Petty Officer Steve Lehmann in New Orleans said a helicopter with a pollution officer on board was headed out Thursday to determine where the oil was coming from.
Shares in Royal Dutch Shell PLC fell in European trading early Thursday after it reported a "light sheen" of oil about one mile long and ten miles wide in the general area of two of its production facilities.
The company says the source of the sheen is unknown and it had no indication the sheen originated from its wells, about 130 miles southeast of New Orleans.
Shell sent a response ship to the area as a precaution.