Prosecutors say Assange also published on WikiLeaks emails from a data breach of an American intelligence community consulting company by a hacker affiliated with LulzSec and Anonymous, another hacking group.
Assange's lawyer, Barry Pollack, said in a statement that "the Government's relentless pursuit of Julian Assange poses a grave threat to journalists everywhere and to the public's right to know."
"While today's superseding indictment is yet another chapter in the US Government's effort to persuade the public that its pursuit of Julian Assange is based on something other than his publication of newsworthy truthful information," he added, "the indictment continues to charge him with violating the Espionage Act based on WikiLeaks publications exposing war crimes committed by the US Government."
Assange was arrested last year after being evicted from the Ecuadorian Embassy in London and is at the centre of an ongoing extradition tussle over whether he should be sent to the US.
The Justice Department has already charged him with conspiring with former US Army intelligence analyst Chelsea Manning in one of the largest compromises of classified information in US history.
Prosecutors say the WikiLeaks founder damaged national security by publishing hundreds of thousands of classified documents, including diplomatic cables and military files on the wars in Iraq and Afghanistan, that harmed the US and its allies and aided its adversaries.
Assange maintains he was acting as a journalist entitled to First Amendment protection. His lawyers have argued the US charges of espionage and computer misuse were politically motivated and an abuse of power.
The allegations in the new indictment centre on conferences as far back as 2009, in locations including the Netherlands and Malaysia, at which prosecutors say he and a WikiLeaks associate sought to recruit hackers who could locate classified information, including material on a "Most Wanted Leaks" list posted on WikiLeaks' website.
According to the new indictment, he told would-be recruits that unless they were a member of the US military, they faced no legal liability for their actions.
At one conference in Malaysia, called the "Hack in the Box Security Conference," Assange told the audience, "I was a famous teenage hacker in Australia, and I've been reading generals' emails since I was 17."
- AP