The official, who asked for anonymity when discussing the case, said U.S. authorities had already sent Britain a provisional arrest warrant regarding Assange's extradition to the United States.
But within 60 days from Thursday, when British police bundled Assange out of the Ecuadorean embassy in London where he had taken refuge seven years ago, U.S. authorities must submit a formal request outlining all the legal charges Assange would face if he is transferred into U.S. custody.
According to a criminal indictment against Assange which prosecutors in Virginia secretly obtained more than a year ago but only unsealed after Assange's arrest, Assange is charged with conspiring with former Army intelligence analyst Chelsea Manning to gain unauthorized access to a government computer.
The U.S. indictment filed in March 2018 said Assange, in March 2010, engaged in a conspiracy to help Manning crack a password stored on Defense Department computers connected to the Secret Internet Protocol Network (SIPRNet), a U.S. government network used for classified documents and communications.
Assange's contacts with Manning led to one of the biggest ever leaks of classified information as WikiLeaks published hundreds of thousands of U.S. military reports and diplomatic communications.