The report warned that virtual economies pose a real-world tax compliance risk, even if citizens aren't purposefully shielding their money.

"Taxpayers may be unaware that income from transactions using this type of virtual currency may be taxable," the paper said. "Or if they are aware, uncertain on how to characterize it."

In response to the GAO report, the IRS said the agency would evaluate the compliance risks and could provide more information for taxpayers. An agency spokesman declined to elaborate on the status of those guidelines.

Other Washington regulators are trying to address Bitcoin and other digital currency.