Canada's Enbridge Inc., already under fire from U.S. regulators over a massive oil spill two years ago, said on Friday it had shut a key pipeline indefinitely after an oil leak in Wisconsin.

Line 14, a 318,000 barrel per day leg of the major Lakehead System that carries light crude oil from Canada to Chicago-area refineries, was shut after a spill that released an estimated 1,200 barrels of oil, Enbridge Energy Partners said in statement. The cause of the spill was undetermined.

"Enbridge is treating this situation as a top priority," said Richard Adams, vice president of U.S. Operations at Enbridge. "We are bringing all necessary resources to bear."

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While the estimated size of the spill is not particularly large, it comes just weeks after a scathing report from the National Transportation Safety Board over Enbridge's handling of a major leak that spilled more than 20,000 barrel in Michigan in July 2010 – almost two years ago to the day.

The NTSB said its investigation found a complete breakdown of company safety measures, while its employees performed like "Keystone Kops" trying to contain it. The rupture, which went undetected for 17 hours, spilled more than 20,000 barrels of heavy crude into Michigan's Kalamazoo River.

In response to the report, Enbridge said it believed its personnel were trying to do the "right thing" at the time.

On Friday, Enbridge said it had no estimate on when Line 14, a 24-inch diameter pipe that was installed in 1988, could be restarted. In most cases, smaller pipeline leaks can be repaired quickly allowing operations to resume pumping, although regulators may require significant work if they find any cause for alarm.

No injury was reported at the line, which is near Grand Marsh, Wisconsin, Enbridge said.