One of the classified documents seized by the FBI at former US president Donald Trump's Florida residence described the nuclear capabilities and military defenses of a foreign power, The Washington Post reported Tuesday.

The newspaper, citing unnamed sources familiar with the case, said that some of the documents found in Trump's Mar-a-Lago residence were so highly classified that only the president and cabinet or near-cabinet level officials would be cleared to authorize other government officials to access them.

It did not name the country whose defense and nuclear capabilities were cited in the document.

According to the Post, such documents require special clearances on a need-to-know basis, rather than a generic top-secret clearance.

There were no details as to where in the residence, which also serves as a private members club, the highly sensitive material was found, or under what type of security.

Trump is facing mounting legal pressure, with the Justice Department saying top secret documents were "likely concealed" to obstruct an FBI probe into Trump's potential mishandling of classified materials.

When agents searched the Mar-a-Lago resort on August 8, they found material so sensitive that "even the FBI counterintelligence personnel and DOJ attorneys conducting the review required additional clearances before they were permitted to review certain documents," a government court filing said.

The FBI raid came after a review of "highly classified" records that Trump finally surrendered to authorities in January after months of back and forth with the National Archives and Records Administration.

The 15 boxes handed over by Trump were found to contain 184 documents marked as confidential, secret or top secret.

After prompting from the FBI, Trump's lawyer eventually turned over an additional 38 classified documents -- and provided "sworn certification" that they represented the last of the material.

But the FBI went on to uncover "multiple sources of evidence" showing classified documents remained at Mar-a-Lago.