CALGARY, Alberta (Reuters) – Canada on Monday formally invoked a 1977 treaty with the United States to request negotiations over Enbridge Inc’s Line 5 pipeline, escalating a long-running dispute over one of Canada’s major oil export pipelines.

Line 5 ships 540,000 barrels per day of crude and refined products from Superior, Wisconsin, to Sarnia, Ontario, but the state of Michigan wants it shut down over worries that a leak could develop in a four-mile section running beneath the Straits of Mackinac in the Great Lakes.

Enbridge ignored Michigan’s order to halt operations earlier this year. The sides are embroiled in a legal battle, and took part in court-ordered mediation. The government of Canada has been pushing counterparts in the United States to intervene to help keep the pipeline open.

In a letter to the federal judge presiding over the case, Gordon Giff, legal counsel for the Canadian government, said Canada had formally invoked Article Six of the 1977 Transit Pipelines Treaty.

The treaty, designed to stop U.S. or Canadian public officials from impeding the flow of oil in transit, has never been invoked before.

Canada’s foreign ministry and the U.S. State Department did not immediately respond to requests for comment.

Enbridge spokeswoman Tracy Larsson said Michigan had let parties know it is not committed to further mediation.

“We greatly appreciate the efforts of ‘Team Canada’ – from the Government of Canada to the provinces of Ontario, Quebec, Alberta and Saskatchewan for their commitments and efforts to keep Line 5 open,” she said in an email.

Additonal reporting by Steve Scherer in Ottawa; Editing by David Gregorio