Oil giant Shell says a leak in a North Sea pipeline is under control.
A leak in a flow line leading to the Gannet Alpha oil platform, 180 km (113 miles) east of Aberdeen, was found on Wednesday.
The company said the size of sea surface affected was estimated to be some 31 km (19 miles) by 4.3 km (2.5 miles) at its widest point - it is unclear how much oil has escaped.
Conservationists had urged the company to give more details about the spill.
Scotland's First Minister Alex Salmond said that although the amount of oil released was "pretty limited" the Scottish government treat any incident in the North Sea with "great seriousness".
Mr Salmond said: "We should obviously put things into context. We are probably dealing with a leak here of 100 tonnes or so of oil, and if you take the Gulf of Mexico that was half a million tonnes.
"But nonetheless, a pipe line leak is a serious matter so the first thing to establish is whether the mechanisms are in place to control it as quickly as possible.
"That is being done at the present moment, and of course and investigation will follow to establish the cause."
A spokeswoman for Shell said the flowline on the seabed had been isolated and depressurized and the leakage of oil had been "considerably reduced".
The company said the sheen from the spill was currently moving west from the oil field.
The spokeswoman added: "Our current expectation is it will be naturally dispersed through wave action and will not reach shore. The weather currently is southerly winds of 25 to 30 knots and the sea state is some two-metre waves.
"We have deployed a Remote-Operated Vehicle to do inspection checks and monitor the subsea leak."
Earlier Greenpeace expressed concern about the lack of information coming from Shell.
RSPB Scotland said it was also worried about young seabirds in the area, dispersing from breeding colonies.
Director Stuart Housden said: "We need to know the type of oil, how much has been released, the local weather conditions and the readiness to deal with any problems.
"These data are vital for proper contingency planning."
Juliet Swann from Friends of the Earth Scotland said Shell had still not released enough information and she questioned why it had taken so long for them to give more details about the spill.
The Gannet oil field reportedly produced about 13,500 barrels of oil per day between January and April of this year.
The field is co-owned by Esso, a subsidiary of US oil firm Exxon but operated by Shell.