Ariana Grande and Big Sean
Big Sean is upset with Justin Bieber for getting flirty with Ariana Grande during her concert at The Forum in InglewoodGetty

American rapper Big Sean is reportedly upset with Canadian singer/songwriter Justin Bieber for getting close with Ariana Grande during a surprise appearance at her Inglewood, California gig.

On 8 April, the Baby singer performed two songs alongside the Nickelodeon star - Love Me Harder and As Long as You Love Me. Bieber also went solo to sing his new hit single Where Are U Now.

During the performance, the Boyfriend singer got flirty with Grande while on stage.

"Sean was not playing, he's pissed. He gets that they're performing and all that but he did not like the way Justin was touching Ariana. He thinks he needs to be taken down a notch," a source told Hollywood Life.

"He's calmed down now but this isn't over. He's super protective of Ariana and he's not going to let her get manhandled like that. It's not even a jealousy thing, he thinks Justin was disrespecting Ariana to touch her that way when she clearly didn't want him to, it wasn't part of their plan, that's why it looked so awkward. All her fans know she's with Sean, why would they want to see Justin all over her like that, it's just not cool," the insider added.

This comes after the Dark Sky Paradise hinted that he is ready to take his romance with Grande to the next level.

During an interview with Larry King, rapper Big Sean revealed that his relationship with the 21-year-old pop star had gotten "pretty serious".

Big Sean and Grande confirmed their relationship in August 2014 after they were pictured holding hands backstage at the 27th annual MTV Video Music Awards.