U.S. regulators warned a subsidiary of Enbridge in January that it wasn't adequately monitoring the corrosion of a Michigan pipeline that leaked an estimated 3.8 million litres of oil into the Kalamazoo River this week.

The Pipeline and Hazardous Materials Safety Administration at the U.S. Department of Transportation informed the Canadian company's chairman, Terry Gill, in writing on Jan. 21. It said that Enbridge's monitoring of corrosion in Line 6B was out of line with U.S. regulations.

Months later, the pipeline spilled several million litres of oil into Talmadge Creek, a tributary of the Kalamazoo River.

An Enbridge spokesperson contacted by The Associated Press on Thursday declined to comment on the letter.

It's not the first time the company has caught the attention of a government agency in the United States. Since 2002, Enbridge and its affiliates have been cited 30 times by the Department of Transportation's regulatory branch.

The governor of Michigan has been putting pressure on Enbridge to clean up the oil, which had travelled 55 kilometres downstream by late Wednesday.

The U.S. Environmental Protection Agency has taken control of clean-up efforts. But Michigan Governor Jennifer Granholm said the U.S. government needed to provide more help, and called the work undertaken by Enbridge and the EPA "wholly inadequate."

The oil has flowed through a city of 52,000 called Battle Creek, which is located 175 kilometres west of Detroit, and is making its way towards Kalamazoo, the area's largest city.

There were conflicting reports from government officials as to whether the oil had surpassed a dam, which some had hoped could help contain the spill.

If the oil were to reach Lake Michigan, which lies 128 kilometres away, it would represent a "tragedy of historic proportions," Granholm said.

Enbridge president and chief executive Patrick Daniel has said the company is "committing to cleaning up anything and everything" that the oil touches outside of the pipeline.

He has also said the company had made "significant progress" in the race to contain the spill, but stopped short of providing information on how long it will take to clean up the mess.

Enbridge has so far put 300 people to work on the Michigan river, placing nearly 4.3 kilometres of protective booms on the river by the end of Wednesday. It will also dig up its pipe to determine what caused the leak.

The company detected the oil leak in its pipeline on Monday in Michigan's Talmadge Creek.

Before the leak could be repaired, the oil flowed into the Kalamzoo River and Morrow Lake.

With files from The Associated Press and a report from CTV's Janet Dirks