WikiLeaks said Saturday the Twitter accounts of four supporters had been subpoenaed in connection with an espionage investigation into the whistleblowing website led by a secret US grand jury.
WikiLeaks, which began releasing 251,287 US diplomatic cables in November, added it had reason to believe Facebook and Google had also received court orders requesting details on users.
"Today, the existence of a secret US government grand jury espionage investigation into Wikileaks was confirmed for the first time as a subpoena was brought into the public domain," WikiLeaks said in a statement.
WikiLeaks said legal action taken by micro-blogging website Twitter "revealed that the US State Department has requested the private messages, contact information, IP addresses, and personal details of Julian Assange and three other individuals associated with WikiLeaks, in addition to WikiLeaks’ own account, which has 634,071 followers".
It did not name the three other people, but Icelandic lawmaker Birgitta Jonsdottir said on her Twitter feed on Saturday US authorities had asked Twitter to submit her account details and personal information.
"Just got this: Twitter has received legal process requesting information regarding your Twitter account in (relation to wikileaks)," the media freedom champion posted overnight.
"The request for my tweet information is from the US department of justice." "The request for information from twitter is also for my personal information not just tweets," she said.
Jonsdottir -- a close associate of WikiLeaks who in September suggested Julian Assange step aside as the site's spokesperson because of rape allegations against him -- said she discussed the request with Iceland's justice minister.
"He is looking into the case of demands of DoJ (department of justice) wanting my twitter details," she posted shortly after 1100 GMT Saturday.
She explained she had 10 days to stop the legal process and stressed the US Department of Justice, not Twitter, was to blame.
WikiLeaks said Saturday it was "opposing the subpoena order and is currently taking action to instruct US lawyers".
It urged Twitter to protect its users' private information and stressed that other than Assange, the three people whose accounts had been subpoenaed had never worked for the site.
"Two were instrumental in helping WikiLeaks bring the Collateral Murder video -- which showed a US helicopter crew celebrating as they gunned down civilians -- into the public domain," WikiLeaks said.
The April 2010 release of the classified video, which shows a US Apache helicopter strike in Baghdad that killed several people in 2007, helped push WikiLeaks into the global spotlight.
The site has since angered the Pentagon by posting in July 2010 secret documents on the war in Afghanistan, followed in October by a massive leak of so-called "Iraq war logs".
Its November release of US diplomatic cables has embarrassed governments worldwide and prompted many calls for WikiLeaks to face legal action.
The site has also faced financial pressure when credit card giants Visa and Mastercard said they would stop facilitating donations to the website.
"Having tried to silence WikiLeaks by pressuring Paypal, Visa and Mastercard to cut off funds, the US government is now intruding into the private lives of some of WikiLeaks most high-profile supporters," WikiLeaks chief Julian Assange said Saturday.
Assange, the 39-year-old Australian who is the public face of WikiLeaks, is currently on bail in Britain facing extradition proceedings to Sweden on charges of sexual assault.