The spill is near the East Harbor Marina in the Patapsco River and originated from a Johns Hopkins Hospital facility in East Baltimore, authorities said.
A spokesperson for Johns Hopkins told CNN the spill was caused by two diesel tanks being overfilled and that it notified the Maryland Department of Environment immediately.
"On Wednesday, during routine fuel delivery by a third-party vendor, two diesel tanks which supply power for backup generators for our patient care facilities at Johns Hopkins in East Baltimore were accidentally overfilled causing overflow," the spokesperson said.
Johns Hopkins is "actively working with local, state, and federal authorities to support the response in the Baltimore Inner Harbor," the spokesperson told CNN early Thursday.
Johns Hopkins first reported the spill around 11:00 a.m. Wednesday and initially estimated it at 100 gallons, according to a news release from Maryland Governor Wes Moore and Baltimore Mayor Brandon M. Scott.
The fuel contains a red dye and has stained a section of the water roughly 100 by 250 yards near the South Central Avenue Bridge, the release said.
"The water is red due to dye in the diesel fuel. There is no impact to drinking water in the area," the release from the governor and the mayor said.
Moore said on social media that he was at the scene of the spill near Fells Point, a historic waterfront in Baltimore and a popular tourist destination.
The US Coast Guard is leading the clean-up effort, which involves nearly a dozen state and city agencies.
The Coast Guard is working with crews using oil absorbent materials and skimmers to remove fuel from the water. "Containment and cleanup efforts will continue through the night," officials said.
"Responders have deployed pollution response equipment, including containment boom, and are actively working to remove all recoverable product from the waterway," the Coast Guard said in a statement.
CNN has reached out to Johns Hopkins and the Coast Guard for further details about the status and scope of the spill. Johns Hopkins Hospital is the flagship of Johns Hopkins Medicine, the university's renowned medical school.
(The-CNN-Wire & 2025 Cable News Network, Inc., a Time Warner Company. All rights reserved.)