Daniel Ellsberg, who leaked the Pentagon Papers – which detailed secrets about US policy during the Vietnam war – and became one of the world’s most famous whistleblowers, has terminal cancer and expects to die within months, he has announced on Twitter.

Ellsberg, 91, tweeted late on Thursday that doctors have diagnosed him with inoperable pancreatic cancer after he underwent a scan and magnetic resonance imaging (MRI) for an unrelated, “relatively minor” medical issue.

He had been given three to six months to live, more or less, he said, though the cancer had not caused him any early symptoms, as is typical with the illness.

The former US government analyst added that he had opted against chemotherapy, and was assured “of great hospice care when needed”.

“Right now, I am not in any physical pain,” Ellsberg’s tweet continued, adding that after a 2021 hip replacement surgery he feels the fittest he has in years. “My cardiologist has given me license to abandon my salt-free diet of the last six years. This has improved my life dramatically: the pleasure of eating my former favorite foods! And my energy level is high.”

The documents Ellsberg became globally known for leaking were referred to as the Pentagon Papers, which outlined American involvement in Vietnam between 1945 and 1967. They revealed that successive White House administrations realized the US could not win the war there.

More than 58,000 Americans had died and 304,000 were wounded at the end of the war in 1975. Meanwhile, nearly 250,000 South Vietnamese military members had been killed, along with up to 1 million guerrillas from North Vietnam and the Viet Cong. Additionally, more than 2 million civilians from North and South Vietnam, Laos and Cambodia had died amid the fighting.

The New York Times first reported on the Pentagon Papers, and the Washington Post and other publications followed up. The Nixon administration obtained a court order temporarily halting the coverage after arguing it threatened national security. But the judges on the US supreme court at that time later voted 6-3 to strike down that order, ruling that the constitution severely limits the government’s ability to exercise “prior restraint” of materials the nation’s free press wishes to publish.

Coverage of the Pentagon Papers won the New York Times a Pulitzer for public service. The entire saga was retold in the Oscar-nominated film The Post, which Steven Spielberg directed. The British actor Matthew Rhys portrayed Ellsberg.

Ellsberg was tried on charges of espionage, conspiracy and stealing government property, and he was threatened with up to 115 years in prison. But the case against him was dismissed due to government misconduct. That misconduct included a burglary of Ellsberg’s psychiatrist’s office, which contributed to the scandal that forced Nixon to resign as president in 1974.

Ellsberg served with the US Marine Corps in the 1950s before going to Vietnam as a civilian defense department analyst who studied tactics to counter insurgents. The Rand Corporation employed him when he leaked the Pentagon Papers.

More recently, he expressed support for the US soldier Chelsea Manning – who leaked Iraq and Afghanistan war records – and Edward Snowden, who blew the whistle on National Security Agency phone and internet surveillance.

Ellsberg told the Guardian in 2021 that he “never regretted” leaking the Pentagon Papers.

“Of course you’d be radically transparent about this too,” the American author Jonathan Katz wrote in a reply to Ellsberg’s tweet Thursday. “Very sorry to hear the news but glad you are still with us and making the best of the time you have left.”