Fuel tanks ruptured on stranded 17,000-tonne oil rig
The Transocean Winner was carrying 280 metric tonnes of diesel when it grounded on Lewis.
The fuel tanks of a 17,000-tonne oil rig that ran aground on Lewis have ruptured.
The Transocean Winner broke free from its tug during a storm on Monday and grounded near Carloway.
A salvage team was winched aboard on Tuesday and found two of its four fuel tanks had breached.
The vessel was carrying 280 metric tonnes of diesel when it grounded - around 300,000 litres - but it is unclear how much has leaked into the sea.
A spokesman for the Western Isles Emergency Planning Coordinating group said a "low level of pollution" had been detected.
They added: "The preliminary visit to the rig revealed some evidence that a number of the diesel storage tanks on the rig may have breached and that it the likely cause of the low level of pollution detected.
"The council, alongside the Animal and Plant Health Agency, is monitoring any potential animal health issues."
Poor flying conditions made it impossible for the salvage team to continue their inspection on Wednesday.
A temporary 300-metre exclusion zone has been established around the Transocean Winner and the emergency towing vessel Herakles is standing by.
Environment secretary Roseanna Cunningham said: "We continue to closely monitor this ongoing situation and the response that is being coordinated by the UK Government, which has responsibility for managing such incidents.
"The news that some of the fuel tanks appear to have breached is obviously concerning, especially given the proximity of the grounded rig to the beach and the presence in the wider area of important marine habitats and species.
"Although the diesel is expected to rapidly disperse in the current sea conditions, the Environment Group, which is chaired by Marine Scotland, has put in place measures to swiftly identify any potential environmental impact on this precious and fragile marine habitat.
"This whole incident raises serious questions about why this rig was being towed through Scottish waters when such stormy conditions were forecast, and the deputy first minister has been in direct contact with the UK Government about this very point."
Friends of the Earth Scotland said it could create a "serious problem" for vital wildlife, tourism and fishing in the area.
Director Dr Richard Dixon said: "If the diesel oil leaks into the environment, the clamour for answers as to why such a risky trip was attempted will grow much louder.
"Lessons from this incident will need to be learnt, and quickly, with further decommissioning of North Sea rigs expected and climate change expected to create more powerful storms and difficult seas.
"The rapid reinstatement of the Emergency Towing Vessel that was based at Stornoway could go some way to alleviating the concerns of communities in this area."