The cause of the death was not immediately known.

The Paris-born designer gained fame for dressing such Hollywood celebrities as Natalie Wood, Grace Kelly, Marilyn Monroe and Gene Tierney, who later became his wife. (The two divorced in 1952, but he continued his designer career in Hollywood until the late 1960s.)

In 1960, then-first lady Jacqueline Kennedy appointed him her official designer.

He made more than 100 dresses in the first year of the Kennedy administration and 300 more thereafter, helping the first lady to gain recognition as one of the best-dressed women in Washington.

He worked closely with her on her personal clothing style, complementing her dresses and outer wear with matching hats, furs, gloves, shoes, and handbags.

"I had not merely selected from my current collection,I had created a concept for her," the designer would later recall. "I talked to her like a movie star, and told her that she needed a story, a scenario as First Lady."

He said he had told Jacqueline Kennedy that he wanted her to set fashion trends rather than follow them.

Cassini was born Oleg Loiewski in 1913 to a family of Russian noblemen.

His mother was Countess Marguerite Cassini, his father Count Alexander Loiewski.

He was raised and educated in Italy, where he attended first an English Catholic school and then a fine arts academy.

Cassini's private life became the focus of gossip columns in the early 1950s when he pursued actress Grace Kelly. However, she rejected him in favor of Prince Ranier of Monaco, whom she later married.

During World War II, the designer served in the US Cavalry Corps, but then returned to New York and opened his own fashion house.

He continued his design work in New York for several decades, pioneering several successful clothes lines, including ready-to-wear sheath dresses, knitted suits, jackets and cocktail dresses, men's shirts and jackets as well as perfume lines.