An operation to remove diesel from an oil rig grounded on the Western Isles has caused no pollution, experts have said.

The Transocean Winner rig ran aground on Lewis 11 days ago and on Thursday night, the owners apologised to the local community as they admitted they are not ready to refloat the giant structure.

The semi-submersible remains at Dalmore beach on the western side of the island after it was blown ashore in severe weather when it detached from its tug en-route from Norway to Malta.

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The rig had 280 tonnes of diesel on board and two of its four fuel tanks were damaged in the incident, resulting in the loss of up to 53,000 litres of fuel, but most of which is thought to have evaporated.

An operation to remove the remaining fuel continues.

A Maritime and Coastguard Agency spokeswoman said on Friday: "Transocean has confirmed they started the operation yesterday to remove some of the diesel oil in the pontoon. This will continue today.

"A verification flight by the Maritime and Coastguard Agency's counter-pollution team has been carried out following that work and there is no evidence of pollution in the area."

A public meeting was held on Thursday evening at nearby Carloway, where various officials updated residents on the ongoing salvage operation.

Dave Walls, operations director with Transocean, thanked the community and emergency services for their help.

READ MORE: Exposed: the safety record of stranded oil rig company

"The other thing I would like to do is to apologise for the disruption to your daily lives," he added.

The meeting was told good progress is being made in the operation to refloat the structure, but it remains "tricky to predict" when that can actually take place.

Mr Walls said much has already been accomplished by the 15 people who are now working aboard the rig, with emergency generators, pumps and internal cameras among the systems already up and running.

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Photo: Andrew Milligan/PA Wire

But with the compressors needed to refloat the structure set to arrive on the island by ferry on Friday, officials cannot yet state when the rig will actually be removed from the beach.

Mr Walls said: "We need to get ourselves in the position where we're ready to float, and we're not there yet.

READ MORE: Exposed: the safety record of stranded oil rig company

"Once we're ready to float we then need the ideal conditions to float - and that's a suitable weather window, no wind, the right tide.

"Everything needs to be just right because we get one opportunity to do it right."

He vowed that no trace of the rig would be left when the salvage operation - including a sweep of the seabed - is complete.

A Marine Accident Investigation Branch investigation has been launched into the incident.