Two advocacy groups say the U.S. Coast Guard doesn't have an adequate strategy for dealing with oil spills in the channel linking Lakes Huron and Michigan.
The Coast Guard approved a plan last year for responding to spills in northern Michigan, including the Straits of Mackinac.
A section of Enbridge Inc.'s Line 5 oil pipeline runs beneath the straits.
In a federal suit filed Wednesday, the Environmental Law & Policy Center and the National Wildlife Federation say Coast Guard leaders have acknowledged they're unprepared to clean up a spill in open Great Lakes waters.
They say Line 5 cannot legally operate without a valid response plan.
Coast Guard spokesman Lt. Paul Rhynard declined comment on the suit but said the Guard has a "robust" process for approving oil spill plans.