Sienna Miller has blasted US censors for giving romantic epic CASANOVA a restrictive 'R' rating when films that terrify kids get PG-13 releases.

Bosses at the Motion Picture Association of America gave the film an 'R' (forbidding people under the age of 17 from viewing it unless accompanied by an adult) because of a banquet scene which hints at Heath Ledger's Casanova character having illicit oral sex with an underaged admirer.

But Miller is appalled by the harsh rating because films that should be given more restrictive releases are passed by the MPAA.

She says, "I think it's a pretty harsh restriction when you can go and see terrifying horror films in this country (America). That seems really silly to me.

"War of the Worlds was PG©13 and that was terrifying. I took a nine-year-old to see it and I had to take him out of the theatre because he was terrified."