Thousands of gallons of diesel oil lost from grounded Transocean Winner
More than 12,000 gallons of fuel (56,000 litres) are believed to have been lost from the grounded drilling rig Transocean Winner.
A UK government official overseeing the operation to remove the rig from Dalmore on Lewis said most of the escaped diesel oil had evaporated.
The structure which broke free from its tow in stormy weather was carrying more than 60,000 gallons of fuel.
No oil has been seen on the surface of the sea.
The rig ran aground off the Hebridean island on Monday morning while being towed off west Scotland.
It is understood it was being towed from Norway to Malta before later being moved to a yard in Turkey where it was to be scrapped.
The structure is expected to remain at the site for some time and heavy rain and high winds have hampered efforts to fully assess the damage.
Rig inspection was not possible again on Wednesday due to poor visibility.
The diesel oil leaked after two of four fuel tanks on the rig were breached.
Hugh Shaw, the Secretary of State's Representative for Maritime Salvage and Intervention, who is overseeing the salvage operation, told BBC Radio 4's Today programme it was thought that this happened during the grounding.
he said the loss was "obviously a concern" but posed an "extremely low risk" to the environment.
The Maritime and Coastguard Agency (MCA) said a maximum of 53 tonnes was thought to have been lost from the damaged tanks, adding that monitoring of the rig's condition was continuing.
It said: "There is no pollution detected in the area of the rig and the MCA continue to monitor for any pollution.
"As well as continual observations on scene, the MCA counter-pollution surveillance aircraft yesterday flew over the location and surrounding area to make careful observations for pollution around the rig and the nearby coast and sea and no sheen or sign of pollution has been found."
The MCA added: "Diesel is a light and non-persistent oil which presents much lower environmental risks than heavy black crude oil."
Poor weather conditions, including low cloud, have so far prevented a team from being airlifted by helicopter on to the rig to make further assessments of the structure.
Mr Shaw told BBC Scotland that the rig's ballasts appeared to have been damaged.
He said "a lot of work" had still to be done to make sure the rig was stable and safe enough to be moved.
Transocean Winner ran aground during a period of high winds.
Charts produced for surfers suggest there was an intense storm with sea swells in the range of 30ft while the rig was being towed off west Scotland.
The tug towing Transocean Winner is understood to have got into difficulty on Sunday evening before the tow line broke in the early hours of Monday.
The drilling rig ran aground at Dalmore at about 07:30 on Monday.
South Devon-based Magicseaweed.com produces swell charts which are used by surfers around the world.
It said storms of Sunday's intensity could occur at any time of the year, but were most frequently in winter.
The orange and red colours on the charts indicate swell of up to 30ft.
The Met Office had issued a yellow "be aware" warning of high winds for Sunday.
Deputy First Minister John Swinney has contacted UK government officials to question why the rig was being towed when stormy conditions were forecast.
BBC News Scotland online asked the rig's owner, Transocean, why the towing operation was attempted on Sunday.
Responding, the company said: "During severe weather, the Transocean Winner lost its tow and subsequently grounded off the Western Isles of Scotland. No rig personnel are at risk.
"Transocean is working closely with authorities to resolve the situation as quickly as possible.
"Further information will be released as it becomes available."
Transocean added: "An initial inspection of the rig has revealed that two fuel tanks in the starboard pontoon have been breached. Transocean is working closely with authorities to resolve the situation as expeditiously and safely as possible."
Rig grounding timeline
Events that led to the Transocean Winner coming ashore on Lewis began on Sunday evening, according to Western Isles MP Angus MacNeil.
18:15 Sunday: Crew of Alp Forward, the tug towing the drilling rig, call for assistance in heavy seas while off the west coast of the Western Isles.
18:30 Sunday: Coastguard emergency towing vessel Herakles begins its journey from Kirkwall in Orkney.
04:15 Monday: Towing line between the rig and Alp Forward breaks.
07:30: Transocean Winner runs aground at Dalmore near Carloway on Lewis.
12:30: Herakles arrives at the scene of the grounding.
The multi-agency Western Isles Emergency Planning Co-ordinating Group (WIEPCG) met on Wednesday afternoon.
A spokesman for WIEPCG said the Scottish Environment Protection Agency (SEPA) was conducting daily shoreline inspections and had indicated that there was no visible sign of pollution.
He said: "Field chemistry staff are also on the island conducting air monitoring in the vicinity on a precautionary basis.
"WIEPCG is continuing to work closely with the Secretary of State's representative's salvage group and the national agencies environment group to protect and ensure public safety. We would again reiterate that people should stay away from the area until such time as we advise."