Michael Douglas and Catherine Zeta-Jones Separate

Michael Douglas and Catherine Zeta-Jones haven't been photographed together since April 22. They've walked red carpets solo, vacationed separately with their kids, and in recent days, Douglas has been relaxing on a yacht off the coast of Sardinia – with Zeta-Jones at home in New York.

So what's really going on with one of Hollywood's most celebrated couples? "Catherine and Michael are taking some time apart to evaluate and work on their marriage," a spokeswoman for Zeta-Jones said in a statement.

Zeta-Jones' statement comes in the wake of reports the couple's marriage was on the rocks, with two of the couple's close confidantes telling WHO: "They're taking a break," one source says.

Neither star has filed for divorce or moved toward a legal separation, sources say.

PHOTOS: Michael & Catherine: Their Marriage In Photos

The A-list pair, who wed in 2000 in a lavish celebration at Manhattan's Plaza Hotel, have faced more than their fair share of ups and downs over the last three years – between Douglas's 2010 cancer battle and Zeta-Jones's struggles with bipolar II disorder. "The stress has taken a toll on their marriage," a friend tells WHO.

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Douglas, 68, and Zeta-Jones, 43, who have two children, Dylan, 13, and Carys, 10, together, decided to spend time apart soon after he came back from the Cannes Film Festival in May. Zeta-Jones had recently returned from a scheduled second round of treatment to monitor her bipolar disorder.

RELATED: Catherine Zeta-Jones Opens Up About Bipolar II Diagnosis

Those close to the couple hold out hope they can work out their differences. Says one family friend: "They want the best for their kids, no matter what happens."

PHOTOS: Michael & Catherine: Their Marriage In Photos

For more inside details of their relationship and the strains that led them to separate, pick up this week's issue of WHO, on newsstands now.