A document revealing a foreign government's military defenses, including its nuclear capabilities, was discovered by FBI investigators who searched the Mar-a-Lago mansion and private club of former US President Donald Trump last month, according to sources speaking to The Washington Post on Tuesday.

According to the report, some stolen documents describe top-secret US operations so highly guarded that many senior national security officials are kept in the know about them.

Only the President, certain members of his Cabinet, or a near-Cabinet-level official could authorize other government officials to know the specifics of these special access programs, according to individuals familiar with the search who spoke on the condition of anonymity to describe sensitive details of an ongoing investigation.

The sources did not identify the foreign country in question, indicate where the paper was discovered at Mar-a-Lago, or provide any information about the probe.

A spokesperson for Trump did not immediately comment. Justice Department and FBI spokespeople declined to comment.

The FBI raided the former President's house in August, seizing over 20 boxes of documents from Mar-a-Lago, including "top secret" classified government materials.

A federal court determined Monday that a special master will be appointed to evaluate the records taken in the Mar-a-Lago raid.

According to a recent story in The New York Times, the United States government discovered more than 300 secret documents at Mar-a-Lago.

According to the article, the secret documents contained information from the Central Intelligence Agency, the National Security Agency, and the Federal Bureau of Investigation.