Justin Bieber was blocked by security when he tried to talk to Rihanna at the Billboard Music Awards on Sunday (May 22).

The 17-year-old singer—who was accompanied to the bash by girlfriend Selena Gomez—was reportedly left red-faced after he was prevented from talking to the "S&M" star after he approached her five times at the prestigious event, which took place at Las Vegas's MGM Grand Garden Arena.

The insider said: "Justin thinks Rihanna is the most beautiful woman and supremely talented.

"He is in awe of her. Justin is just a red-blooded teenager.

"He can't help acting like a love-sick puppy dog when he's around her.

"The pair were chatting as Justin's girlfriend Selena looked on, scowling.

"Then Rihanna got dragged away by a couple of her friends.

"The pair resumed their chat later, but when Justin went to talk to her at the show's climax her security blocked his path, telling him to 'leave it.'"

The "Baby" hitmaker was said to be "mortified" at the rebuff.

He thought he and Rihanna were friends after talking to her at the BRIT awards earlier this year.

The source added to the Daily Mirror newspaper: "Justin looked mortified —he thought they were buddies."

However, the teenage singing sensation appears to be over his disappointment as he has jetted off on holiday to Hawaii with Selena.

The pair were spotted arriving in Maui hand-in-hand yesterday (May 23) after taking a flight from Los Angeles LAX airport, and headed straight to the beach.