NEW YORK (Reuters) - Between 9 and 18 barrels (378-756 gallons) of oil spilled into Lake Michigan from BP Plc's Whiting refinery in Indiana after a malfunction on Monday, the U.S. Coast Guard confirmed on Wednesday.

The spill occurred Monday afternoon at a recently upgraded water treatment plant located in a cove that leads into the lake, the company and the U.S. Environmental Protection Agency said. About 5,000 square yards of water was coated in an oily sheen and oil slicks could be seen on the shore and rocks, BP said in its initial report to the EPA.

The clean-up continued Wednesday as crews vacuumed up 5,200 gallons of oily water from the area, a U.S. Coast Guard spokesman said. Barriers at the mouth of the cove to block the oil from flowing to the lake were replaced after strong winds and waves blew them out of place on Tuesday. The winds blew the oil further into the cove up onto the shoreline, the Coast guard spokesman said.

There is no evidence that oil leaked outside of the containment boom, the EPA said.

(Reporting By Elizabeth Dilts; Editing by Nick Zieminski)