US President Donald Trump has delayed the release of thousands of pages of classified documents about John F. Kennedy's assassination due to pressure from the CIA, FBI and other federal agencies still seeking to keep some final secrets about the nearly 54-year-old probe, media reports said.

US President Donald Trump has delayed the release of thousands of pages of classified documents about John F. Kennedy's assassination due to pressure from the CIA, FBI and other federal agencies still seeking to keep some final secrets about the nearly 54-year-old probe, media reports said.

However on Thursday evening, the President allowed the immediate release of 2,800 records by the National Archives, following a last-minute scramble to meet a 25-year legal deadline, reports The Washington Post. 

After lobbying by national security officials, the remaining documents will be reviewed during a 180-day period.

In a statement released by the White House, Trump said: "I am ordering today that the veil finally be lifted. At the same time, 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. 

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

The records were put online at 7.30 p.m. on Thursday, The Washington Post reported. 

The thousands of field reports, cables and interview summaries from dozens of FBI, CIA and congressional investigators reveal the minutiae of a chase for information that spanned decades and covered continents. 

Usually typed, stamped "Secret" and often annotated by hand, the files are a paper trail of detective grunt work, leads exhausted, dead-ends encountered, sources checked and re-checked.

Many of the files highlight the desperate search for Lee Harvey Oswald's possible connections to communists, Cubans, or both in the months before he shot Kennedy on November 22, 1963.

Oswald, a troubled former Marine who had temporarily defected to the Soviet Union at one point, was killed on November 24, 1963, by nightclub owner Jack Ruby at Dallas police headquarters on live television.

Several show the FBI's often extraordinary efforts to identify suspected communists in the US. 

Some documents also summarised internal discussions within Communist Party meetings after the assassination, discussing whether Oswald was innocent and whether communists would be blamed for Kennedy's death.

The government was facing a Thursday deadline for disclosing the records, and Trump tweeted twice that the documents would be made public.

"The long anticipated release of the #JFKFiles will take place tomorrow," he tweeted on Wednesday. "So interesting!"

Meanwhile, some of the material that assassination experts had been most eager to review was not included in the documents released on Thursday. 

The missing records include a 338-page file on J. Walton Moore, the head of the CIA office in Dallas at the time of the killing, and an 18-page dossier on Gordon McClendon, a Dallas businessman who conferred with Ruby just before he shot Oswald. 

The National Archives has had custody of the records since the Warren Commission published its investigative findings in 1964, reports The Washington Post.

By the early 1990s, only a sliver of the Warren Commission's papers - just 2­ per cent - had been concealed, either partially or in full, according to the National Archives. 

Since then, the archives has made periodic releases of its repository, which totals more than 5 million pages. In a recent article on its website, the archives said that 88 per cent of its documents are fully open; 11 per cent have been released but with redactions; and 1 per cent has been fully withheld.