The Trans Alaska Pipeline shut down on Saturday after a leak was discovered at the intake pump station at Prudhoe Bay, constricting supply in one of the United States’ key oil arteries.

Alyeska Pipeline Service Co., the operator of the 800-mile (1,280 kilometre) line which runs from the Prudhoe Bay oilfield to the tanker port of Valdez, said the leak was discovered Saturday morning. Oil producers are in the process of cutting output to 5 percent of the normal rate of around 630,000 barrels per day.

There is no estimate yet of how long the pipeline — which carries about 12 percent of U.S. oil production — will be shut down or when normal production can resume, said Alyeska spokeswoman Michelle Egan.