By Erwin Seba and Edward McAllister

(Reuters) - An unknown amount of oil leaked from BP Plc's Whiting refinery in Indiana into Lake Michigan after a mechanical glitch on Monday afternoon, the company confirmed on Tuesday.

The discharge had stopped, and the leak was contained, the London-based company said in a statement on Tuesday.

No injuries were reported, and the effect on Lake Michigan was not immediately clear. About 60 percent of the lake is covered in ice, according to the National Oceanic and Atmospheric Administration.

BP has laid down boom on the water to keep the leak from spreading and said the oil was in a cove between the refinery's wastewater treatment plant and a steel mill.

Crews were working on the cleanup on Tuesday. The U.S. Environmental Protection Agency, the Coast Guard and state regulators were at the scene.

Winds were pushing the oil onto the shore, while cold temperatures were causing it to harden into a waxy consistency, making it easier to collect, BP spokesman Scott Dean said.

Dean declined to say how much oil was released.

The largest crude distillation unit at the 405,000-barrel-per-day refinery was back to normal operations after a malfunction led to the leak, Dean said.

The 260,000-bpd crude distillation unit, called Pipestill 12, was the centerpiece of a $4 billion refit of the Whiting refinery completed late last year to run large amounts of oil from Canada's tar sands fields in Alberta.

A crude distillation unit does the initial refining of crude oil coming into a refinery and provides feedstock for all other units at the plant.

The refinery has been increasing the amount of Canadian crude oil running through Pipestill 12 during the first quarter of this year.

Environmental groups opposed the BP upgrade, saying use of the Canadian crude would increase pollution from the refinery into Lake Michigan.

The Whiting refinery is the primary focus of BP's U.S. refining strategy to use only plants in the northern United States that have in easy access to Canadian crude oil. BP sold plants in Texas and California last year as part of the strategy.

BP's American depositary shares were down 1 cent at $46.69 in morning New York Stock Exchange trading. The company's stock was up 0.4 percent in London.

(Reporting by Edward McAlliser and Selam Gerekidan in New York, Arpan Varghese in Bangalore, Erwin Seba and Kristen Hays in Houston and David Sheppard in London; Editing by Lisa Von Ahn and Jeff Benkoe)