One Direction fans are sending death threats to a US boyband suing them for "stealing" their name.

The chart-topping British band are facing a lawsuit from a little-known Californian five-piece who share the same name as them. The US group are reportedly demanding $1 million for alleged copyright breaches.

The American group are now claiming they are being harassed by One Direction fans.

There has been so much hate mail, like people threatening to cut our b***s off," frontman Sean O'Leary told UK newspaper The Sun.

"The one that stands out for me is a girl saying she wanted to kill my father."

One Direction - comprising of Louis Tomlinson, Harry Styles, Niall Horan, Zayn Malik and Liam Payne recently made music history by becoming the first British group to top the US charts with their debut album, Up All Night.

The group were formed on Simon Cowell's UK version of The X Factor, and the US boyband who are suing the group say they are not afraid of the music mogul.

"He's obviously a very smart man... but we are not intimidated. The law isn't a popularity contest. Our name has been infringed," Sean told BBC Newsbeat.

"My dad would always advise me you're at the bottom so there's only one direction to go. And that was the attitude I needed for the band."

Bandmate Scott agreed with the sentiment.

"It was just so awkward that someone with our name was on a game show [The X Factor]. That name means much more to us. It's everything," he said.