"We are going to step up our effort and already are stepping up our efforts on all leaks," Sessions, America's top cop, said at a news conference in response to a reporter's question about a US priority to arrest Assange.
The Justice Department chief said a rash of leaks of sensitive secrets appeared unprecedented.
"This is a matter that's gone beyond anything I'm aware of. We have professionals that have been in the security business of the United States for many years that are shocked by the number of leaks and some of them are quite serious," he said.
"Whenever a case can be made, we will seek to put some people in jail."
Prosecutors in recent weeks have been drafting a memo that looks at charges against Assange and members of WikiLeaks that possibly include conspiracy, theft of government property and violations of the Espionage Act, the Washington Post reported, citing unnamed US officials familiar with the matter.
