Oil and gas giant Total has restricted access to a wellhead platform following difficulties while drilling for gas in the North Sea.
Workers encountered an unexpected flow of gas as they drilled more than 4,000 metres (13,123ft) below the seabed beneath the Elgin B platform.
Part of the well may now have to be abandoned.
Total said the well remains "under full control" at the surface.
It is understood the company has taken a series of precautionary measures while it deals with the incident.
Only staff involved in drilling the well are now allowed onto the wellhead platform, while the wells from Elgin B have been shut down.
A Total spokesman said: "Total has encountered difficulties whilst drilling a well on its Central Graben asset. The well remains under full control at the surface and the situation is under continuous review.
"Wireline measurements carried out inside the drill string indicate some flow exists within the well which means part of the well may need to be plugged and abandoned."
WWF Scotland director Lang Banks said: "This is a worrying development, as this is not the first time Total has had problems at its Elgin-Franklin field.
"Only a few years ago, a leak on its platform complex led to an emergency evacuation and a halt to production for almost a year.
"Protecting staff and the marine environment should always be priority number one for the oil and gas industry.
"Given the previous problems at this site, serious questions need to be asked by the relevant authorities as to what is going on."
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