"We anticipate those charges, but do not have any information from the US as the grand jury proceedings are secret -- and therefore the charges are secret," said Baltasar Garzon, the Spanish former judge who heads Assange's legal team.
"If charges are secret, (Assange) is completely helpless" to plan how to answer the allegations made against him, Garzon told reporters in Quito.
The 41-year-old Assange, an Australian, is holed up in Ecuador's embassy in London and is seeking asylum in the South American nation to avoid extradition to Sweden, where he is accused of sexual assault.
But he also fears being extradited to the United States to face charges over WikiLeaks' publishing of a trove of secret documents, including information relating to the wars in Iraq and Afghanistan, and countless diplomatic cables.
Extradition from Sweden to the United States has been deemed extremely unlikely by several legal experts, who conclude that an extradition order would be easier put into effect directly from the United Kingdom.
"Britain has to agree to his extradition from Sweden, and for that to happen the affair needs to be tried in a British courtroom," said law professor Christoffer Wong from Lund University to national TV station SVT earlier this summer.
Assange's mother Christine met on Wednesday with Ecuadoran President Rafael Correa, who told her "the important thing is for Julian to be assured that Ecuador is considering with great responsibility" his request.
Assange's mother says she is worried her son could face execution if he is extradited to the United States.