Assange has been indicted in the U.S. on 18 charges over the publication of classified documents. Prosecutors say he conspired with U.S. army intelligence analyst Chelsea Manning to crack a password and hack into a Pentagon computer and release secret diplomatic cables and military files on the wars in Iraq and Afghanistan.

Assange argues he was acting as a journalist entitled to First Amendment protection, and says the leaked documents exposed U.S. military wrongdoing. Among the files published by WikiLeaks was video of a 2007 Apache helicopter attack by American forces in Baghdad that killed 11 people, including two Reuters journalists.

But Lewis said Assange was guilty of “straightforward" criminal activity in trying to hack the computer. And he said WikiLeaks' activities created a “grave and imminent risk” to U.S. intelligence sources in Afghanistan and Iraq.

"By disseminating the materials in an unredacted form, he likely put people — human rights activists, journalists, advocates, religious leaders, dissidents and their families — at risk of serious harm, torture or even death," the lawyer said.

Lewis said some informants and others who had been assisting the Americans had to be relocated after the leak, and others “subsequently disappeared."