The British government warned on Monday that several hundred tons of oil may have leaked into the North Sea from a Royal Dutch Shell rig.
The department for energy and climate change said that it estimates that the leak from a flow line at the Gannet Alpha platform off the Scottish coast that began last week could have spilled several hundred tons of oil into the sea.
It said that the leak was small compared to the BP spill in the Gulf of Mexico in 2010, but said that it was still substantial for the UK's continental shelf. The government said that the oil would disperse naturally and was not expected to reach the shore.
It said Britain's offshore oil industry had a strong safety record, "which is why it is disappointing that this spill has happened. We take any spill very seriously and we will be investigating the causes of the spill and learning any lessons from the response to it."
The government said the Maritime and Coastguard Agency, which monitors the waters around Britain, was making twice-daily flights over the area to monitor the situation.
Shell has declined to comment on the volume off the spill. It said in a statement released over the weekend that the spill covered a surface area of 19 miles by 2.7 miles (31 kilometers by 4.3 kilometers) and that the leak was under control.
The oil field, about 112 miles (180km) east of the city of Aberdeen, is operated by Shell and co-owned by Shell and Esso, a subsidiary of the US oil firm Exxon Mobil.
Stuart Housden, director of the Royal Society for the Protection of Birds Scotland, said razorbills, puffins and guillemots that gather in the North Sea in late summer could be at risk.
"We know oil of any amount, if in the wrong place, at the wrong time, can have a devastating impact on marine life," he said.
The Scottish government said that it was working with Shell to monitor the spill and warn local fishing boats about it.