The hundreds of "I Love NY" highway signs that cost taxpayers $8.1 million are illegal, according to the U.S. Department of Transportation, which claims state officials knew the signage violated federal law prior to its installation.

The Federal Highway Administration told Fox News the 514 bright blue placards, which cost roughly $15,000 each, must be modified because they amount to advertisements -- instead of being navigational or safety-related -- and are a dangerous distraction to drivers.

"They do not conform to federal rules," said Doug Hecox, a spokesman for the Federal Highway Administration, a division of the U.S. Department of Transportation.

"The signs have a URL – they're encouraging drivers to go to a website," Hecox said.

The use of signs for advertising purposes has been prohibited since the 1960s. Hecox said the signs in question -- which currently dot roadways across New York -- specifically fail to comply with the Manual of Uniform Traffic Control Devices and the Highway Beautification Act.

The signs, which started appearing in 2014, are on heavily trafficked highways, like the New York State Thruway, and at major entry and ferry points. Many of them appear in groups of five -- sprouting along the road in rapid succession -- and all include a mention of ILoveNY.com and the state's I Love NY mobile app.

The signs remain part of an ongoing dispute between New York state and the federal government. According to multiple press reports, the government last year threatened fines and the potential withholding of federal funds if New York does not comply with the law.

When asked whether New York Gov. Andrew Cuomo knew in advance the signs violated federal law, Hecox replied, "Yes."

A spokesperson from Cuomo's office as well as the New York State Department of Transportation did not respond to requests for comment on the matter.

The state has said it believes the signs are in compliance with the federal manual and state guidelines, according to documents obtained by the USA Today network's Albany bureau.

"The sign designs and spacing seek to impress simple but memorable information for later recall without posing a distraction," Todd Westhuis, acting director of the state DOT's Office of Traffic Safety and Mobility, wrote to the Federal Highway Administration in 2014, USA Today reported.

Cuomo himself has also promoted the signs, saying in his 2014 State of the State address: "The goal is to get people who are on the roads off the roads and into communities and fostering and promoting the economy of the State of New York."

Hecox told Fox News on Tuesday he believes the state is currently "making a good faith effort" to comply with federal rules, saying the New York State Department of Transportation is developing an "experimental proposal" for the Federal Highway Administration's consideration.

"FHWA will review the proposal upon receipt and work with NYSDOT on next steps," he said.

Cristina Corbin is a Fox News reporter based in New York. Follow her on Twitter @CristinaCorbin.