US regulators on Thursday announced a record $13.3m fine against a major television company for airing promotional content without telling viewers it had been paid to air it.

The fine against conservative-leaning Sinclair Broadcasting Group would be the largest ever for a violation of federal laws that require broadcasters to reveal when sponsors pay for content that appears like normal programming, according the Federal Communications Commission.

Sinclair, the largest US broadcast television group, operates nearly 200 local television stations across the country and is currently attempting to buy stations in dozens more markets from Tribune Media Co. -- a deal that requires FCC approval.

Democratic lawmakers have accused the FCC of bias in favour of Sinclair and The Washington Post last year said the broadcaster favoured President Donald Trump's election campaign.

FCC Chairman Ajit Pai and Sinclair deny those allegations.

Looked like local news coverage

According to the FCC, Sinclair aired and supplied other broadcasters with ads that looked like local news coverage about the Huntsman Cancer Institute. The content was dressed up to look independently produced but actually was paid for by the Huntsman Cancer Foundation.

The programming aired more than 1,700 times, and the payment was not disclosed, according to the FCC.

"The FCC's sponsorship identification rules help ensure that consumers know who is trying to persuade them through paid programming," the FCC said in a statement.

The Huntsman Cancer Institute is a cancer research facility in Utah founded by the industrial chemicals magnate Jon Huntsman Sr., whose son Jon Huntsman Jr. is a Republican former diplomat and governor.

The foundation, which raises funds for the institute, said it ended its contract with Sinclair after learning its ads were not properly labeled, according to Bloomberg.

Sinclair has 30 days to contest the fine or pay it.

Last year, Sinclair paid a $9.5 million fine to the FCC over unrelated violations.