The Petroleum Safety Authority (PSA) has confirmed it has launched an investigation into a North Sea leak at the Statfjord field.
Operator Equinor had to halt production on its Statfjord A platform last week after oil was spotted on the surface of the water.
The PSA said that that the location of the leak and the volume of oil which was released is still unknown.
The regulator added that police have been informed about its investigation.
Norwegian energy major Equinor said the issue came to light on May 15 while oil was being loaded onto a shuttle tanker.
The firm said it had taken steps to map the extent of the leak.
The Norwegian share of the field lies in blocks 33/9 and 33/12 in production licence 037, while the UK part is in block 211/25 in licences 104 and 293.
The field was developed with the Statfjord A, B and C production platforms.
The PSA said it would “review and clarify” events, identify the causes and “uncover and describe” the actual and potential consequences of the incident.
Equinor said earlier this month that it would shut down down the Statfjord A platform, 43 years after it produced first oil, but that the other two installations would remain on stream until at least 2025.