A BP refinery has spilled an undetermined amount of crude oil into Lake Michigan in the U.S., according to BBC News.

BP said a malfunction caused crude oil to enter the Whiting refinery's cooling water system, which discharges into the lake.

The Whiting facility is BP's largest U.S. refinery and is in the north-western part of the state of Indiana, near Chicago, Illinois.

The Whiting spill had affected a 2,700ft (820m) stretch of the lake's shore, US officials said, adding that there was currently no threat to wildlife or human health.

BP said the leak occurred late Monday, March 24 afternoon, but that the discharge had now stopped. A clean-up crew had deployed a boom which seemed to have contained the oil, it said.

However, the company said efforts were continuing to remove the crude oil, which had mainly affected a cove near the refinery's waste water treatment plant.

BP was recently granted rights to bid for government oil contracts in the U.S., after being banned from doing so in the wake of the 2010 Deepwater Horizon oil spill in the Gulf of Mexico - the largest in U.S. history.

A few weeks ago, BP announced plans to increase its processing of crude oil at the Whiting refinery, as more oil is sent down from tar sands in Canada.

Two US senators who represent Illinois - Mark Kirk, a Republican, and Dick Durbin, a Democrat - released a statement that said they were concerned about BP's efforts to prevent future spills.

"We plan to hold BP accountable for this spill and will ask for a thorough report about the cause of this spill, the impact of the Whiting Refinery's production increase on Lake Michigan, and what steps are being taken to prevent any future spill," they said in statement.

BP said in a statement that it "will continue to work in full cooperation with the agencies to ensure the protection of personnel, the environment and surrounding communities".