"The United States is deeply concerned about reports of additional charges facing Kian Tajbakhsh," said President Barack Obama's spokesman Robert Gibbs.
Tajbakhsh was sentenced in October to 15 years in prison for his part in unrest that followed polls in June which saw President Mahmoud Ahmadinejad re-elected.
On Monday he was handed further charges of "spying for the George Soros foundation," according to the New York Times, citing a Tajbakhsh family member. The "foundation" refers to Soros' Open Society Institute, a pro-democracy group founded by the financier and philanthropist.
"The charges against Mr. Tajbakhsh are baseless, and his original sentence on October 20 was an outrage," said Gibbs.
"The Iranian government cannot earn the respect of the international community when it violates universal rights and continues to imprison innocent people," he added.
