The US government on Thursday released a mammoth, long-awaited trove of secret files on the assassination of president John F. Kennedy, but withheld others for further review on national security grounds.

In a statement, the National Archives said that on orders from President Donald Trump, it had released 2,891 records related to the November 22, 1963 slaying of JFK in Dallas, Texas.

Kennedy scholars have said that the documents were unlikely to contain any bombshell revelations or put to rest the rampant conspiracy theories about the assassination.

One of the documents included a transcript of a November 24, 1963 conversation with J. Edgar Hoover, who was FBI director at the time.

Hoover said the FBI informed police of a threat against the life of Lee Harvey Oswald the night before Oswald was killed. But police did not act on it, Hoover said.

The Warren Commission, which investigated the shooting of the charismatic Kennedy, 46, determined that Oswald, a former Marine sharpshooter, carried out the Kennedy assassination acting alone.

The released files are vast in number and scope, covering everything from FBI directors' memos to interviews with members of the public in Dallas who came forward trying to provide clues after that singularly unforgettable moment in US history.

Some date into the 1970s and included handwritten official notes which are hard to read.

Trump said in a memorandum that he had agreed to hold back for further review some records relating to the killing.

Administration officials who requested anonymity said the majority of those requests had come from the Central Intelligence Agency and Federal Bureau of Investigation.

"Executive departments and agencies have proposed to me that certain information should continue to be redacted because of national security, law enforcement, and foreign affairs concerns," Trump said.

"I have no choice — today — but to accept those redactions rather than allow potentially irreversible harm to our nation's security," he said.

Trump gave agencies six months — until April 26, 2018 — to make their case for why the remaining documents should not be made public.

"At the end of that period, I will order the public disclosure of any information that the agencies cannot demonstrate meets the statutory standard for continued postponement of disclosure," he said.

Full and unredacted

The 2,891 records approved for release are viewable on the National Archives website, in full and unredacted form.

"The president wants to ensure that there is full transparency here," an official said, but "there does remain sensitive information in the records."

This includes, for example, the identities of informants and "activities that were conducted with the support of foreign partner organizations, either intelligence or law enforcement," the official said.

The Warren Commission's formal conclusion that Oswald killed JFK has done little to quell speculation that a more sinister plot was behind the murder of the 35th US president.

Hundreds of books and movies such as the 1991 Oliver Stone film "JFK" have fed the conspiracy industry, pointing the finger at Cold War rivals the Soviet Union or Cuba, the Mafia and even Kennedy's vice president, Lyndon Johnson.

The release of the documents is in compliance with an October 26, 1992 act of Congress which required that the assassination records held in the National Archives be released in full and unredacted 25 years later.

Kennedy assassination experts eagerly awaited the opportunity to look at the files but sought to tamp down expectations.

Gerald Posner, author of "Case Closed," which determined that Oswald did indeed act alone, said people who think the files will "have the solution to the case that everybody can settle on" are going to be disappointed.

"No one's going to abandon their belief in a conspiracy because the release of the files doesn't prove it," Posner told AFP. "They'll just say it must have been destroyed or hidden."

Experts agreed, however, that the documents may shed some light on an intriguing chapter in Oswald's life — his trip to Mexico City about seven weeks before the slaying where he is known to have met with Cuban and Soviet spies.

The CIA and FBI may be blocking the release of certain documents to hide their own failings, said Larry Sabato, a professor of politics at the University of Virginia and the author of "The Kennedy Half Century."

"They had every indication that Oswald was a misfit and a sociopath," he said.

But neither agency informed the Secret Service, which is charged with protecting the president, he said.

Oswald defected to the Soviet Union in 1959 but returned to the United States in 1962.

He was shot to death two days after killing Kennedy by a nightclub owner, Jack Ruby, as he was being transferred from the city jail.