California must obtain approval from the U.S. Environmental Protection Agency before enforcing new rules to require ships to burn cleaner diesel fuel as they enter and dock at the state's ports, a federal appeals court ruled on Wednesday.

The California Air Resources Board had sought to implement stricter air pollution controls within 24 miles of the state's coastline. The rules would require ships to use fuel in their auxiliary engines that emits lower levels of sulphur oxide, nitrogen oxide and particulate matter.

But the 9th U.S. Circuit Court of Appeal reinstated a lower court's injunction barring the state from enforcing its Marine Vessel Rules, finding that the Clean Air Act preempts them.