(CNN) -- A Shell-operated oil drilling platform in the North Sea has been shut down while authorities work to fix a leak in a storage pipe, the company said Friday.
Shell said in a statement that a "light sheen" of oil on the surface of the sea alerted authorities to the problem at the Gannet Alpha platform about 180 kilometers (about 112 miles) east of Aberdeen, Scotland.
The statement did not indicate when the leak was thought to have occurred or how much oil was thought to have escaped from the leak.
"We have stemmed the leak significantly and we are taking further measures to isolate it," the statement said. The platform where the leak occurred is jointly owned by Shell and Exxon, and has been in production since 1993.
The company said a remotely operated vehicle was deployed to check for a "subsea leak" after the oil sheen was noticed in the area. "The subsea well has been shut in, and the flow line is being de-pressurized," Shell said in its statement.
Ben Ayliffe, senior climate campaigner for Greenpeace International, said they will assess the situation and send monitoring crews if needed.
"What's interesting here is that this is a mature and well-developed oil field that we've been repeatedly assured is safe," Ayliffe said from Britain. "And yet it shows that accidents do happen, and there is an inherent risk involved here."
Juliet Swann, of environmental campaign group Friends of the Earth Scotland, described the spill as "hugely disturbing".
"Any spill, however small, should serve as a warning sign and encourage us to look to a clean, renewable energy future, rather than continuing to invest in dirty oil," she said in a statement.
Shell's statement said safety and environmental agencies in Britain have been informed of the leak.
A Scottish government spokeswoman confirmed the authorities were aware of the situation and "in close contact with key organizations including Shell. We will continue to monitor the situation and update ministers."
CNN's Ben Brumfield, Alden Mahler Levine and Brian Walker contributed to this report.