Royal Dutch Shell PLC said Friday it has closed a valve from which oil was spilling into the North Sea.
The company said this was a "key step" in stopping the leak at its Gannet Alpha platform, the worst North Sea oil spill in more than a decade.
Shell said it will now monitor the flowline to make sure the valve remains sealed.
Glen Cayley, technical director of Shell's exploration and production activities in Europe, said divers sealed off the valve in a "careful and complex operation."
"But we will be watching the line closely over the next 24 hours and beyond," he said.
"Our next task is to remove the residual oil from inside the depressurized flowline, and that will take time."
About 1,300 barrels of oil has spewed into the sea since a pipeline was found to be leaking Aug. 12, the company said, although it claimed that after is shut the well, only one barrel a day had been leaking from the installation to control the build-up of pressure in the pipeline.
The Maritime and Coastguard Agency said the spill, 110 miles (180 kilometers) east of Aberdeen, Scotland, has caused a sheen covering an area of 6.7 square kilometers (2.5 square miles).