Just days after Justin Timberlake and Jessica Biel announced their split, there is already speculation that the singer-actor's Friends with Benefits costar, Mila Kunis, was the cause of the break-up.

Not true, says a source close to Timberlake, 30.

"Mila had nothing to do with their break-up," the source says. "In fact, no one came between them. They decided their relationship ran its course and it was time to move on."

But now that he is a single man – and Biel, 29, is stepping out as a single woman – has Timberlake already moved on with Kunis, 28?

"They're not in a relationship," the source says. "They star in a movie called Friends with Benefits but they're just friends. That's it. They are not together."