US President Donald Trump has ordered the release of thousands of secret John Fitzgerald Kennedy assassination documents, but yielded to pressure from the FBI and CIA to block the release of hundreds more for further review.

Key points: Some 300 of the 3,100 secret JFK files are being placed under review for six months

Some 300 of the 3,100 secret JFK files are being placed under review for six months The FBI and the CIA reportedly requested that Mr Trump withhold the files

The FBI and the CIA reportedly requested that Mr Trump withhold the files The decision to withhold the files further has already re-ignited conspiracy theories

The National Archives subsequently published 2,800 previously secret documents, but the remaining 300-plus files will be placed under a 180-day review so government agencies can again scrutinise the remaining documents and justify why any should be withheld, officials said.

In a memo to government agency heads, Mr Trump said the American people deserved as much access as possible to the records.

"Therefore, I am ordering today that the veil finally be lifted," he wrote, adding that he had "no choice" but to accept the requested redactions for now.

Mr Trump will impress upon federal agencies that JFK files should stay secret after the six-month review "only in the rarest cases", the White House said.

Loading

It said the remaining records with redactions would be released "on a rolling basis" in the coming weeks.

"The President wants to ensure there is full transparency here and is expecting that the agencies do a better job in reducing any conflicts within the redactions and get this information out as quickly as possible," one official said.

Another official said: "There does remain sensitive information in the records that could compromise the identify of informants or intelligence operations."

Congress had ordered in 1992 that all records relating to the investigation into Kennedy's death should be open to the public, and set a final deadline of October 26, 2017, for the entire set to be made public.

White House officials said the FBI and CIA made the most requests within the Government to withhold some information.

Withholding documents fuels suspicions of a 'smoking gun'

JFK and Jackie Kennedy arrive in Dallas, Texas, hours before he was killed. ( Cecil Stoughton/White House )

The last batch of assassination files includes more than 3,100 documents — comprising hundreds of thousands of pages — that have never been seen by the public.

No blockbusters had been expected in the last trove of secret files regarding Kennedy's assassination on November 22, 1963, however, the move by Mr Trump to withhold the files under pressure from intelligence agencies has already reignited conspiracy theories.

Lee Harvey Oswald as he is shot by Jack Ruby in the basement of Dallas Police Headquarters on November 24, 1963. ( Ira Jefferson Beers Jr/Dallas Morning News )

"As long as the Government is withholding documents like these, it's going to fuel suspicion that there is a smoking gun out there about the Kennedy assassination," said Patrick Maney, a presidential historian at Boston College.

Thousands of books, articles, TV shows and films have explored the idea that Kennedy's assassination was the result of an elaborate conspiracy.

None have produced conclusive proof that Lee Harvey Oswald, who was fatally shot two days after killing Kennedy, worked with anyone else, although they retain a powerful cultural currency.

"My students are really sceptical that Oswald was the lone assassin," Mr Maney said.

"It's hard to get our minds around this, that someone like a loner, a loser, could on his own have murdered Kennedy and changed the course of world history. But that's where the evidence is."

Kennedy's assassination was the first in a string of politically motivated killings, including those of his brother Robert F Kennedy and civil rights leader Martin Luther King Jr, that stunned the US during the turbulent 1960s.

JFK remains one of the most admired US presidents in history.

AP/Reuters