One Direction's Irish star Niall Horan.Getty Images

The 21-year-old singer partied with the 'Yeah 3X' hitmaker in Las Vegas and is believed to have been advised by his One Direction bandmates - Harry Styles, Liam Payne, Zayn Malik and Louis Tomlinson -

to ditch the 25-year-old singer.

A band insider told the Star on Sunday newspaper: "The rest of the lads - especially Harry and Louis - are really unhappy about Niall's new friendship with Chris.

"Niall has been advised to dump Chris ASAP - but Chris is still texting him every day. It's causing real friction behind the scenes."

Chris was sentenced to five years of probation for assaulting his former girlfriend, Rihanna, in 2009 and recently served a stint in jail for breaking the terms of his parole.

The insider added: "Niall and Chris are getting on really well and he thinks the other lads need to just let him do his thing.

"But the other lads don't see it that way at all. This is the guy who beat up Rihanna. He's not exactly got the kind of image One Direction want to be associated with."

But Niall may be forced to end his friendship with Chris after the band's boss, Simon Cowell, "freaked out" over the pictures.

The insider said: "Simon was freaked out by the photos of their escapades. They have to be careful what image they put out to their American female fans."