Russia feared an 'irresponsible' US general could launch a revenge missile strike in the wake of John F Kennedy's assassination, while Dallas police knew about the threat to kill Lee Harvey Oswald, it has emerged.

Thousands of previously classified files relating to the assassination of the US President were released on Thursday shedding new details on the case, although Donald Trump bowed to pressure from the CIA and FBI by blocking the release of hundreds of more sensitive records.

The US president said he had made the decision because of national security concerns, but allowed the release of 2,800 other files.

"I have no choice," the US president said in a memo, citing "potentially irreversible harm" to national security if he were to allow all records out now. He was placing those files under a six-month review.

Historians warned that the files were unlikely to contain any bombshell revelations or put to rest the rampant conspiracy theories about the 1963 assassination.

• Four unanswered questions from the JFK conspiracy

The documents approved for release show federal agents madly chasing after tips, however thin, in the days after the November 22, 1963, assassination and juggling rumours and leads worldwide.