Private letters revealing opera star Maria Callas' desire for revenge after she was jilted by billionaire Aristotle Onassis are being auctioned in Rome.

Strong international interest is expected for the sale of six letters written in the dying days of Callas' love affair with the Greek shipping tycoon.

It's like I've been given a huge smack and I can't breathe any longer

Maria Callas

Sent to her voice teacher Elvira De Hidalgo in the late 1960s, they unmask a bitterness against Onassis and Jacqueline Kennedy - the woman he was to marry.

Days after Onassis wed the widow of assassinated US President John F Kennedy in October 1969, Callas wrote: "It's cruel, it isn't true - both should pay and both will pay, you'll see."

The former first lady became known as Jackie O

Callas died alone, depressed and unfulfilled, in Paris in September 1977, aged 53.

Onassis, who had once given her a $1m diamond ring, died in March 1975.

Auctioneer Christie's is putting 11 lots of her letters and photographs under the hammer with a reserve price of more than $15,000 (�9,500).

Heavy bidding is expected from Callas' homeland of Greece.

Written as her nine-year affair with Onassis was collapsing, Callas' letters also touched on the disillusion she felt and her battle to lose weight.

The diva also struggled to keep her divine voice

She wrote: "I'm doing all right under the circumstances, but it's like I've been given a huge smack and I can't breathe any longer.

"I've received three telephone calls (from Onassis). One I didn't take - the other two I did and it was a disaster.

"As I told you, he's lost it and that's something that's really horrible for me to accept.

"I'm in Paris and I'm going to try to put a little order in my oh-so-painful head. I'll just try to survive these coming few months."

Their affair continued after the marriage, but it was to prove the most humiliating period of Callas' tragic life.

As she struggled to hold on to her lover, the woman dubbed the "high priestess of opera" was also fighting to retain her voice.