Paul Bettany says having young children "scares the s**t" out of him.

The 'Broken Lines' actor - who has two children, Stellan, eight, and four-month-old Agnes, with wife Jennifer Connelly, as well as raising Kai, the actress' 14-year-old son from a previous relationship - loves being a father but admits he is always nervous about The Early Years of his kids' lives.

He said: "I have a very small baby. Kids are great, great - but the first three years they just scare the s**t out of you."

Paul - who has been married to Jennifer for eight years - also admits he finds it "heartbreaking" when work means he has to be separated from his family for lengthy periods of time.

He said in an interview with The Guardian newspaper: "It's so weird, so difficult and occasionally heartbreaking. I have a rule that we'll never go more than two weeks without seeing each other."

The 40-year-old star also admitted he understands why people are fascinated by his marriage because it has lasted such a long time compared with other Hollywood unions.

He said: "The thing is, we've been married for eight years, which is like 50 years in Hollywood years. So I suppose it suddenly becomes really intriguing - 'How do they do it?' "