David McCallum, who became one of TV's biggest stars of the 1960s playing Russian spy Illya Kuryakin in "The Man from U.N.C.L.E." and then won over a new generation of fans on the popular "NCIS" series decades later, has died at 90, NCIS said on Monday.

"We are deeply saddened by the passing of David McCallum and privileged that CBS was his home for so many years. David was a gifted actor and author, and beloved by many around the world,' NCIS said on social media.

Variety reported that he died of natural causes.

The Scottish-born son of two musicians had an acting career spanning seven decades that dated back to his student days in the 1950s at London's Royal Academy of Dramatic Art, where one of his classmates was future star Joan Collins.

He launched his career with supporting parts in a number of British films, including "A Night to Remember" in 1958, where he played Harold Bride, radio operator on the doomed Titanic. He gained the attention of American audiences with his small but pivotal role as one of the prisoners of war plotting a mass breakout from a German prison camp in the 1963 World War Two classic "The Great Escape."

The film featured a star-studded international cast including Steve McQueen, Richard Attenborough and James Garner. During filming, McCallum introduced his wife, Jill Ireland, to co-star Charles Bronson, whom she married after leaving McCallum.

McCallum also guest-starred in a number of American TV shows including "The Outer Limits" and the legal drama "Perry Mason," where he did a comic turn as a hapless, unlucky-in-love Frenchman. In 1964, he appeared in the pilot of a spy series starring American actor Robert Vaughn as "The Man from U.N.C.L.E." McCallum's part, in which he spoke only a few lines, was to prove his launchpad to international fame.

The show was designed as a showcase for Vaughn as the dashing Napoleon Solo, battling nefarious agents from THRUSH. McCallum played fellow U.N.C.L.E agent Illya Kuryakin. Sporting a blond Beatle haircut and cloaked in mystery and sex appeal, he quickly became a hit with fans and was elevated to co-star, alongside Vaughn.

The role earned McCallum two Emmy nominations and the status of a pop culture idol. MGM, which produced the show, said he attracted more fan mail than any other star in the studio's celebrated history, although McCallum later insisted that "Vaughn got as much as I did."

In a 2016 interview with Britain's Mirror newspaper, McCallum said his fans were so ardent that during a walk in Manhattan's Central Park while the programme was airing, a crowd gathered around him.

"Then people started pushing, and from the back came two mounted policemen, who had to lift me up. We trotted out of the park," he said.

Discussing the appeal of the show, which aired for four seasons, McCallum told the Radio Times in 2015: "It came on at a time when there was tremendous anguish with the Vietnam War and concerns about the Cold War. It was a difficult time in America and a story about two agents, one American and one Russian, who seemed to be very friendly and able to work together in spite of all the anguish, caught on with the public."

McCallum reunited with Vaughn in a 1983 TV movie, "Return of the Man from U.N.C.L.E," and a 1986 episode of "The A-Team," titled "the Say U.N.CL.E. Affair." Vaughn died in 2016.

In subsequent years, McCallum remained busy, especially on TV, starring in the British series "Colditz" from 1972 to 1974 and "Sapphire & Steel" from 1979 to 1982. He also appeared as a guest on a number of popular American television shows, including "Hart to Hart," "Matlock," "Murder She Wrote," "Law and Order" and "Sex and the City."

A new brush with fame awaited when McCallum took on the role of medical examiner Donald "Ducky" Mallard on the CBS military detective show "NCIS." It was one of US television's most popular programmes during a run exceeding 19 years.

McCallum, who told New Zealand TV Guide it was his "best role ever," immersed himself in the part by studying forensic medicine, speaking at pathologists' conventions and even attending autopsies.

"I've had an incredible life," McCallum told the Mirror. "I can sit here for a week and talk to you about the past and 99.9 percent will be positive."

He was the father of five children with Ireland and his second wife, Katherine Carpenter, whom he married in 1967.

Even with his success on "NCIS," in which he appeared into his 80s, McCallum never quite escaped the aura of the character that made him famous.

The lead investigator in "NCIS," played by Mark Harmon, is asked in one episode what McCallum's character looked like as a younger man.

"Illya Kuryakin," he replies. (Reuters)