The company said it lost power at a pump station near Fairbanks during the testing of a fire command system. As a result, relief valves opened and crude oil spilled into a secondary containment system.
"The pipeline is currently shut down and the North Slope producers have been prorated to 16 percent," the company explained.
The 800-mile pipeline from Prudhoe Bay to Valdez, Alaska, had a design capacity of 2 million barrels per day in the 1970s, though production from the region has declined since the late 1980s.
British oil giant BP is the largest shareholder of the pipeline.
Alyeska said it estimated there were "several thousand barrels" of oil spilled as a result of the power failure. The containment area, the company said, has the capacity to hold more than 100,000 barrels of oil.
The pipeline operator said it activated a team to assess the situation.