The three who were arrested while hiking in July on the border with Iraq "are innocent young people, who should be released by the Iranian government. Their release should be expedited," said White House spokesman Robert Gibbs.
Tehran's chief prosecutor Abbas Jaffari Doulatabadi said earlier Monday that investigations were continuing against the three, Shane Bauer, 27, Sarah Shourd, 31, and Josh Fattal, 27, the official IRNA news agency reported.
A statement on their case is to be made in the near future.
"The three Americans arrested near the border of Iran and Iraq are facing charges of spying and the inquiry is continuing," the Iranian prosector said.
Gibbs added that the United States had not heard confirmation about the charges from Swiss diplomats, as Switzerland represents US interests in Iran due to the absence of Iran-US ties.
At the State Department, spokesman Ian Kelly said: "If it is true that they have been formally charged, we would find this outrageous. And, of course, the families would find it devastating."
Family and friends of the three have said they were hiking in a mountainous border region in northern Iraq near a famous waterfall when they unintentionally strayed into Iran.
A Swiss diplomat visited the three Americans in Iran's notorious Evin prison last month, according to their families, who said the trio were "in good physical shape."
Earlier Secretary of State Hillary Clinton said there was no evidence for Iran to charge the three detained hikers.
"We believe strongly that there is no evidence to support any charge whatsoever," Clinton told reporters during a visit to Germany for celebrations commemorating the fall of the Berlin Wall.
State Department spokesman Ian Kelly added here: "If it is true that they have been formally charged, we would find this outrageous. And, of course, the families would find it devastating."
