CHICAGO (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 per cent 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 United States 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.

Mr 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, Mr Dean said.