Nearly 3000 previously-unseen documents from a collection of long-secret John F Kennedy assassination files have been released by the United States National Archives.

But in a last minute hitch, President Donald Trump agreed to withhold hundreds, even thousands, of other documents following furious lobbying by the CIA, FBI and other US government agencies.

He has given those agencies until March next year to mount their arguments for continued secrecy and set an absolute deadline of April 26, 2018 for the release of all documents unless a compelling case is made to do otherwise.

“I am ordering today that the veil finally be lifted,” Mr. Trump said, but added: “I have no choice — today — but to accept those redactions rather than allow potentially irreversible harm to our nation’s security.”

JFK experts believe the 2891 files that were released will provide insight into the inner workings of the CIA and FBI. But they stress that it will take weeks to mine the documents for potentially new and interesting information.

At least one of the documents, a 1975 “study of fact”, details multiple CIA attempts to assassinate foreign leaders. It was part of a line of enquiry into whether JFK’s death was planned by Cuba or the USSR.

A 1975 file notes that Bobby Kennedy, the slain president’s brother and US attorney general, was “unhappy” with attempts on Cuban president Fidel Castro’s life, according to The Guardian. 

A 1992 law required all government records related to the assassination to be “publicly disclosed in full” within 25 years. The deadline was midnight Thursday, US time.

Mr Trump ordered a 180-day review of the remaining files be held so that government agencies can look again at the remaining documents and justify why any should be withheld, officials said.

The officials, who briefed reporters on condition of anonymity, described Mr Trump as reluctant to agree to agency requests to hold back thousands of documents but in the end felt he had no choice but to agree to their entreaties.

“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.

One of the FBI files released today. Photo: Archives.gov

Why are they becoming public now?

In October 26, 1992 President George H W Bush signed a law requiring that all documents related to the assassination be released within 25 years, unless the president says doing so would harm intelligence, law enforcement, military operations or foreign relations.

Will there by any bombshells?

Slim chance, according to the judge who led the independent board that reviewed and released thousands of the assassination documents in the 1990s.

The files that were withheld in full were those the Assassination Records Review Board deemed “not believed relevant,” Judge John Tunheim of Minnesota told The Associated Press.

But Judge Tunheim said it was possible the files contain information the board didn’t realise was important two decades ago.

What will the files show?

Some of the documents are related to Harvey Lee Oswald’s mysterious six-day trip to Mexico City right before the assassination, scholars say.

Oswald said he was visiting the Cuban and Soviet Union embassies to get visas, but much about his time there remains unknown.

Other files scholars hope will be released in full include an internal CIA document on its Mexico City station, and a report on Oswald’s trip from staffers of the House committee that investigated the assassination.

Files pertaining to gunman Lee Harvey Oswald. Photo: Archives.gov

The backstory

The release of the files comes almost 54 years after the assassination of President Kennedy on November 22, 1963 in Dallas, Texas.

Kennedy was travelling that day in the backseat of an open convertible alongside his wife, Jackie, when three shots rang out about 12.30 pm in Dealey Plaza.

Texas Governor, John Connally, riding in the front seat with his wife Nellie, was also wounded, but would later recover.

Controversially, investigators would later conclude Kennedy and Connally were struck by the same bullet. Skeptics have long mocked this “magic bullet” theory; even Connally believed he’d been struck separately.

Mrs Kennedy, according to the report of a special commission of investigation, “saw the President’s skull torn open” by the second bullet that hit him.

She later testified to the commission that she cried out, “Oh, my God! They’ve shot my husband. I love you, Jack.”

Former US Marine, Lee Harvey Oswald, was arrested in Dallas just hours after the shooting and charged with the president’s murder.

He was said to have fired down on the motorcade from the nearby Texas Book Depository as the presidential motorcade went past.

Oswald proclaimed his innocence, saying he was a “just a patsy”.

Accused assassin Oswald was shot dead while being escorted by police. Photo: Getty

Two days later, while he was in police custody, the 24-year-old ex-Marine was gunned down by Dallas nightclub owner Jack Ruby.

In September, 1964, a seven-member commission established by Kennedy’s successor, Lyndon Johnson, and headed by Chief Justice Earl Warren, found that the assassination was the work of Oswald alone. It ruled out any domestic or foreign conspiracies.

It also said Ruby acted alone. He died in jail in 1967.

– with AAP