The discharge from the USCG cutter Mackinaw happened Tuesday afternoon near the Ambassador Bridge, which connects Detroit and Windsor, Ontario.
The Coast Guard said the ship's crew secured the source of fuel and conducted initial cleanup operations onboard. Crewmembers also deployed a boom around the fuel to prevent further spread. A contractor is expected to finish cleanup efforts.
"We take our role as environmental stewards seriously, and any accidental discharge of fuel is regrettable," Capt. Eric Johnson, chief of the Coast Guard 9th District Incident Management Branch, said in a statement. "We are already at work mitigating any potential impacts."
The cause of the discharge is under investigation, Johnson said, and the Coast Guard wants to ensure that another spill doesn't happen.
Ice breaking work is keeping Coast Guard crews busy on the Great Lakes, and the Mackinaw was cleared Tuesday to leave for an ice breaking operation.