A one-by-10-mile oil sheen has appeared in the Gulf of Mexico, and Shell Oil, one of several companies operating in the area, said it has “activated” a vessel with skimming and boom capabilities.

Shell said there was no indication that the sheen, in the central portion of the gulf between the company’s Mars and Ursa production areas, came from its platforms. Other companies also have wells in the area.

Shell notified the National Response Center and called on the Louisiana Responder, which is owned by the Marine Spill Response Corp.

Shell said its “priority is to respond proactively, safely and in close coordination with regulatory agencies.” It asked flights in the area to help monitor the sheen.

The sheen appeared shortly before the anniversary of the April 20, 2010, blowout on a BP well that triggered the largest offshore spill in U.S. history. It also comes as Shell is preparing to drill in the Chukchi Sea off Alaska.

The federal Bureau of Safety and Environmental Enforcement “is taking steps to identify the source of a sheen reported today in the central Gulf of Mexico, and will continue to coordinate with the U.S. Coast Guard to respond to and monitor any potential pollution,” agency spokesman T. David Smith said in a statement.