The former military analyst's release of thousands of documents to US media in 1971 revealed that successive United States administrations had lied to the public about the war.

The leak changed public perceptions of the conflict and was recounted in the 2017 Hollywood thriller "The Post," which detailed the nail-biting behind-the-scenes story of the publication of the papers by the Washington Post.

"On February 17, without much warning, I was diagnosed with inoperable pancreatic cancer," Ellsberg, 91, said in a statement on Twitter Thursday.

"I'm sorry to report to you that my doctors have given me three to six months to live," he added.

Ellsberg wrote that he has chosen not to do chemotherapy as it offers no promise.

"I have assurance of great hospice care when needed," he added.

Ellsberg was a government consultant when he leaked 7,000 classified pages which determined -- contrary to the public assertions of US government officials -- that the Vietnam conflict was unwinnable.

The New York Times published excerpts until the administration of President Richard Nixon obtained a court injunction barring the newspaper from continuing to do so on national security grounds. The Washington Post then took up the mantle.

Ellsberg was charged under America's espionage act but the case ended in a mistrial in 1973 after illegal evidence gathering by the government came to light.

"When I copied the Pentagon Papers in 1969, I had every reason to think I would be spending the rest of my life behind bars. It was a fate I would gladly have accepted if it meant hastening the end of the Vietnam War, unlikely as that seemed (and was)," Ellsberg wrote in his statement Thursday.

"Yet in the end that action -- in ways I could not have foreseen, due to Nixon's illegal responses -- did have an impact on shortening the war," he added.

Ellsberg, a staunch anti-nuclear weapons campaigner published a massive tome about the nuclear threat seen from the inside, titled "The Doomsday Machine: Confessions of a Nuclear War Planner," in 2017.