Nicole Kidman is deeply in love, but not happy

The pair married in Sydney in June of last year but their newlywed bliss was short lived, with Urban checking into rehab for a 90 day stint just four months later.

The actress says that despite her strong feelings, love is no guarantee that the relationship will last.

She told US magazine Interview: "You never know where something's going.

"My husband and I are committed to each other and deeply in love. That's how I would put it. We are working on staying in that place and hopefully we will for the rest of our lives."

Told by the magazine interviewer that she and Urban appeared happy together, she replied: "I don't ever say that.

"Partly because interviews are done in advance, and you never know where something's going.

"People's lives together are complicated and beautiful and that's what they should stay. They're very quiet. Complicatedly quiet. They don't need to be broadcast."

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Kidman also hinted she may be looking to quit her successful Hollywood career.

She said: "I've found other things in my life that can absolutely fulfil me. That's been a necessity because a lot of my life has been spent living for my work."

The 40-year-old star, who has an adopted daughter Bella, 15 and 12-year-old son Conor with former husband Tom Cruise, recently spoke of her hopes having a baby with Urban.

In a recent interview with Vanity Fair, in which she revealed she suffer a miscarriage at 23, Kidman said: "I'm yearning to have one. I think I would be very sad if I wasn't able to have a baby.

"Keith knows I want one, and he has been getting there slowly. My husband really wants a couple of years, he said, 'Where I just have you.' So it's kind of romantic. That's what we decided."

Meanwhile, the slender Australian star is bulking up for her new silver screen role in The Reader.

She has already put on 10lb and is planning to double her weight gain in the coming weeks, according to sources.

A movie insider reveals: "The part she plays demands that she gets fatter throughout the movie as her character slips into middle age."

She stars as Hanna Schmitz in the adaptation of the bestselling German novel by the same name, which deals with the emotional repercussions of the Holocaust in post-war Germany.