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A malfunction at BP's Whiting refinery last week caused the spill of an unknown amount of crude oil into Lake Michigan, according to media reports.

The spill happened on Monday, March 24, when a technical problem caused crude oil to enter the refinery's cooling water system, from where the oil was discharged into the lake. The Whiting refinery in Indiana is the largest BP refinery in the United States.

According to Reuters, the processing unit where the malfunction occurred had recently been upgraded. It is estimated that the amount of oil spilled is about 500 gallons, CBS reported, quoting a local source. The company said that the spill had been contained by the evening of the next day but there has been no official confirmation of the amount of the discharge.

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Officials announced that the oil spill had affected a 2,700ft section of the lake's shore close to the refinery's wastewater treatment plant but said that there was no risk to human health or wildlife at the moment, according to BBC News. BP said that a cleanup operation had been launched to remove the crude oil. The company confirmed that it was fully cooperating with state agencies to make sure the environment, local communities and its staff are protected. But the incident raises questions over whether BP is fully capable of doing so, with the latest spill occurring just days after BP had its ban on entering new contracts in the United States lifted.