Mila Kunis reportedly "hasn't let her guard down" around Ashton Kutcher yet.

The pair have been friends since starring in the US TV series That 70s Show. In recent months there has been speculation that their relationship has become more serious. Although it has been rumoured that they could even be moving in together, Mila is believed to be reluctant to rush into anything.

"She's a smart girl and hasn't totally let her guard down yet," a source told British magazine Heat. "She's still being cautious about their relationship. But Ashton's house is very plush she loves being there. And she's finding it hard to deny she's smitten."

Ashton who split from his wife Demi Moore last year following allegations he was unfaithful is apparently eager for Mila to move into his Hollywood Hills home.

He is believed to be besotted with the 28-year-old actress.

"Mila isn't like the typical LA girl she's really down to earth and isn't impressed by wealth or fortune," the source added. "They've been friends for years, so they're surprised how quickly they've fallen for each other."