The United States government has stepped up its intimidation of whistleblowing media organisation WikiLeaks by forcing several of its supporters to appear before secret grand jury hearings in Alexandria, Virginia.

Those subpoenaed have been targeted for their connections to Bradley Manning, the US army private suspected of leaking thousands of secret documents which were later published by WikiLeaks.

Manning, who is yet to face trial, has been in jailed for more than a year.

David House, a member of the Bradley Manning Support Network, was forced to appear before the secret hearing on June 16, Bloomberg.com said that day.

He told CNN.com he had refused to answer any questions beyond his name and address, citing his right to silence to avoid incriminating himself.

An unknown person who appeared at the grand jury on May 11 also refused to answer questions, Glenn Greenwald said at Salon.com on June 9.

House and others forced to testify are not suspected of aiding Manning in his alleged leaking. The subpoenaing of activists is part of a broader pattern of harassment against supporters of WikiLeaks and whistleblowers in general.

Others who have received subpoenas include Manning's ex-boyfriend Tyler Watkins and a cryptography expert at Princeton, Nadia Heninger, who is linked to WikiLeaks activist Jacob Appelbaum, Greenwald said.

Several potential future witnesses told Greenwald they too intended to exercise their right to silence.

Greenwald said the next step could be prosecutors offering immunity from prosecution to the witnesses, leaving them in a position where they must testify or go to jail.

“But at least some of those witnesses ... intend to refuse to answer questions anyway, risking an almost-certain finding of contempt of court, which typically carries jail terms as a means of forcing testimony,” Greenwald said.

House said in a June 15 statement: “The Administration’s goal is to force these individuals to testify against this media organization in an attempt to cast its publications and those of its media partners ... as acts of espionage.”

House — along with others — has been the target of frequent harassment by authorities. He said: “The government has also violated my Fourth Amendment rights by executing a warrantless seizure on my laptop in an attempt to identify, target and ensnare Cambridge-based supporters of WikiLeaks.”

The grand jury was set up to investigate possible violations of the US Espionage Act after WikiLeaks began releasing thousands of secret US embassy cables in December 2010.

The investigation has sought to find an incriminating connection between Manning and WikiLeaks editor-in-chief Julian Assange.

Despite finding no evidence so far, the government has continued its aggressive pursuit of its goal to criminalise WikiLeaks and deter others from following its example.

Manning's supporters believe his well-documented mistreatment in jail was aimed at inducing him to incriminate Assange.

[For more information on Manning, and the persecution of his supporters, visit www.bradleymanning.org .]