X Factor USA judge Demi Lovato says Simon Cowell "gets on [her] nerves".

The two judges sit alongside Britney Spears and L.A. Reid on the coveted panel of the hit TV talent show and have come to blows in recent weeks.

On Wednesday, Lovato criticised Cowell's boyband Emblem3, commenting after their performance: “I just don't understand what Simon is doing with you guys. Every week I know you are going to be standing next to each other singing a song, but people are going to like you because you are good looking. I don't think this is genuine at all.”

Before Lovato could get another word in Cowell snapped back: "Demi, you criticising me for doing a bad job is quite ironic," and pointed out that he has sold over 300 million records.

Speaking to Extra, Lovato said of the on-screen clashing with Cowell: “He gets on my nerves. I would talk to anybody that way if they were talking to me that way. I don’t care who you are. I don’t care how many records you sold. I’m going to tell you what I think if you are going to bite my head off.”

Things also got personal this week after contestant Tate Stevens sang a version of Keith Urban's Somebody Like You.

Lovato gushed: "What I think is so amazing is that you dedicate every song to your wife and one day when I get married I want my husband to love me as much as you love your wife, she is a very lucky girl."

Cowell retorted: "Demi I think that might be a bit of a tall order actually," before the pop songstress hit back: "At least I am going to be married one day, at least I'll get married one day."