Prosecutors are believed to be preparing or considering charges against WikiLeaks and its founder, according to US media reports, and Jeff Sessions said officials had "already begun to step up our efforts" to crack down on leaks.
Mr Assange is wanted in Sweden over a sexual assault allegation dating back to 2010, but he fears he will be extradited to the US for questioning over the activities of WikiLeaks if he complies with Swedish authorities.
However, Home Office sources have told The Guardian that Sweden's existing arrest warrant would be the UK government's first consideration if Mr Assange left the embassy.
Last month WikiLeaks released nearly 8,000 documents that it says reveal secrets about the CIA's cyber-espionage tools for breaking into computers, mobile phones and even smart TVs.
Mr Assange has not left Ecuador's embassy, tucked behind Harrods in London's Knightsbridge, since he walked through the door seeking political asylum in June 2012.
According to reports on Thursday, authorities are preparing to seek his arrest.
The Justice Department has been unable to build a case against Mr Assange, or any of the more high-profile figures who have leaked to the organisation, until now. Lawyers have found a way around first amendment concerns in the prosecution of Mr Assange, sources tell CNN.
But Mr Assange's lawyer, Barry Pollack, said he had not been made aware of any developments.
He added: “The Department of Justice should not be treating the publication of truthful information as a reason for a criminal investigation of the publisher.
“Democracy has always depended on journalists being able to inform the public of what their government is doing.”
Additional reporting by agencies