It said "radicalisers appear to be particularly vulnerable in the area of authority when their private and public behaviours are not consistent".

It continued: "Some of the vulnerabilities, if exposed, would likely call into question the radicaliser's devotion to the jihadist cause, leading to the degradation or loss of his authority."

A potentially damaging piece of evidence would show a militant "viewing sexually explicit material online or using sexually explicit persuasive language when communicating with inexperienced young girls," it said.

The leaked document is the latest in a cascade of revelations from Snowden revealing the NSA's large-scale online spying.

The six individuals targeted were seen as radical Muslims giving inflammatory speeches, but were not described in the document as linked to terror plots.

The Huffington Post withheld their identities and locations.

All six live outside the US, though one was described as a "US person," meaning he is either a citizen or has permanent resident status.

Through electronic spying, the agency had found sexually explicit information about at least two of the people targeted, some of which was gleaned through FBI surveillance, said The Huffington Post.

One of those targeted was described as a "respected academic" who has promoted the idea that "offensive jihad is justified". The spy agency concluded he is potentially vulnerable because of his alleged "online promiscuity" and that he publishes "articles without checking facts", according to the report.

US intelligence agencies did not deny the report or question the validity of the document.

Shawn Turner, spokesman for the director of national intelligence, said: "Without discussing specific individuals, it should not be surprising that the US government uses all of the lawful tools at our disposal to impede the efforts of valid terrorist targets who seek to harm the nation and radicalise others to violence."

The methods discussed are reminiscent of tactics used by the FBI, the NSA and other agencies in the past to tarnish civil rights leaders, anti-war activists and labour organisers.

When an inquiry by lawmakers in the 1970s exposed the domestic surveillance, Congress adopted new limits on spying powers and created a foreign surveillance court that is supposed to review requests for eavesdropping on communications between a foreign suspect and a US resident.

Jameel Jaffer, of the American Civil Liberties Union, said: "This report is an unwelcome reminder of what it means to give an intelligence agency unfettered access to individuals' most sensitive information.

"One ordinarily associates these kinds of tactics with the secret police services of authoritarian governments.

"That these tactics have been adopted by the world's leading democracy is truly chilling."