Justin Bieber does not think that any fight with the hero of Mission: Impossible movies will be a cruise in the ring.

This is why the singer, who issued a challenge to Tom Cruise, 56, on Sunday (June 9) to a showdown in the mixed-martial-art octagon, said on Wednesday that it was made only in jest.

According to portal TMZ, he told journalists outside his wife Hailey's fashion-line office that he had "seen an interview with him and he was just on my mind".

"I don't know why.

"It was just a random tweet. I do that stuff sometimes."

Asked if he could take on Cruise, who does his own stunts in films, Bieber, 25, admitted that the actor would "probably whoop my ass in a fight" and that he would need "to get into some good shape" if a bout were to be held.

"I'm really skinny right now," said Bieber.

"I think he would probably be out of my weight class. He's got that dad strength."

Cruise has not commented on Bieber's dare.