How you get on US terrorism watch list, but have no way of finding out why

US authorities can place Americans and foreigners on a terrorist watch list indefinitely on the basis of vague rules without sound evidence, according to a leaked government document.

Once blacklisted, individuals have no way of finding out why they are deemed suspicious and even dead people's names remain on the list, under secret guidelines set out last year by the National Counter-Terrorism Centre(NCTC), which were published for the first time by The Intercept news website.

The rules introduced by President Barack Obama's administration represented an expansion of the government's power when it came to designating suspected terrorists, laying out broad criteria for adding names to the lists, The Intercept reported.

Individuals added to the watch lists could be banned from flying or subjected to additional searches and security screening at airports and border crossings.

The rules say government agencies can place a name on the watch list based on a "reasonable suspicion" but not on "unfounded suspicion or hunches", according to the 166-page document, entitled March 2013 Watchlisting Guidance.

"Although irrefutable evidence or concrete facts are not necessary, to be reasonable, suspicion should be as clear and as fully developed as circumstances permit," it said.

Social media postings could be considered as supporting evidence, and "should not automatically be discounted", it said.

An agency can propose to add a person to the list based on a single source, "even if that source is uncorroborated".

The watch lists, which became a priority after the attacks of September 11, 2001, have long been criticised as arbitrary and unwieldy.

But US authorities took a tougher approach after al-Qaeda militant Umar Farouk Abdulmutallab tried to blow up an airliner bound for Detroit on Christmas Day 2009, with an explosive hidden in his underwear.

The Nigerian's name appeared on a watch list but he was able board a passenger plane headed for the US, prompting Obama to call for a review of the watch lists.

A civil court case recently revealed that the watch list had grown dramatically in the wake of the Detroit plot, with the government adding 1.5 million names since then.

The court case also showed that when agencies "nominate" individuals for the watch list, other government agencies rarely reject the proposed additions. Only about 1 per cent of more than 460,000 nominations were vetoed last year.

Human rights groups slammed the guidelines, saying the government had created an unaccountable system that allowed authorities to label individuals with little proof.

"Instead of a watch list limited to actual, known terrorists, the government has built a vast system based on the unproven and flawed premise that it can predict if a person will commit a terrorist act in the future," said Hina Shamsi, director of the American Civil Liberties Union's national security project.

Shamsi accused the government of offering virtually no recourse for trying to clear a name.

The leaked document was an exclusive for The Intercept, a news site recently launched by Glenn Greenwald, a former columnist at The Guardian.

Greenwald helped break the bombshell revelations from former intelligence contractor Edward Snowden, whose leaks exposed the US government's far-reaching electronic spying.

Wednesday's report on the watch lists made no mention of Snowden as the source of the NCTC document.