Coast guards say two fuel tanks have been breached on an oil rig that was blown onto a remote Scottish beach, though it's not clear how much diesel has escaped.

The Transocean Winner was being towed when it broke free of its tug and ran aground on the Isle of Lewis off Scotland's west coast on Monday.

The Maritime and Coastguard Agency says the rig is carrying 280 tons of diesel in several tanks.

The agency says salvage workers boarded the rig Tuesday and "discovered that two of the fuel tanks appear to have been breached." Poor weather prevented the workers returning to the rig Wednesday.

Environmental groups have expressed concerns about the accident, and local politicians have called for an emergency towing vessel to be based in the area.