Angelina Jolie, best actress nominee for "Changeling," arrives at the 81st Academy Awards in Hollywood, California February 22, 2009. [Agencies]

Angelina Jolie is trying to lose 21lbs in 21 days.

The 33-year-old actress is reportedly desperate to shed weight for upcoming film 'Salt', in which she plays a CIA assassin who is forced to disguise herself as a man.

A source said: "To play a man convincingly Angelina is adamant she needs to lose her curves."

Friends claim Angelina is so determined to look like a boy she has turned to the Martha's Vineyard Detox Diet to help shrink her already slender frame.

Angelina reportedly started her detox three days before last month's Oscars and is already battling weakness and fatigue.

Another source told Britain's Grazia magazine: "She has been trying to go for the full 21 days but it's tough. She's lost around eight pounds but is now introducing a couple of small meals in between shakes as she was starting to feel weak."

Angelina's partner Brad Pitt - with who she raises six children - is reportedly so fed up with her new diet he stormed out of their rented Long Island mansion last week.

A friend said: "Brad told her that if she was staying on the diet, he was getting out of the way. Then she started screaming at him about a chore.

"He felt that would he did or said would be wrong, so he went to Los Angeles to let off steam with some buddies. He thinks the diet is dangerous and unnecessary."