Ms Saberi was earlier accused of working with an expired press card
The US has expressed dismay after a court in Iran jailed an Iranian-American journalist, Roxana Saberi, for eight years on spying charges.
Ms Saberi, 31, was sentenced after a secret one-day trial in Tehran.
President Barack Obama "is deeply disappointed at this news," his spokesman Robert Gibbs said.
Correspondents say the case will have serious implications for US-Iranian relations at a time when Mr Obama has reached out to the Tehran.
US Secretary of State Hillary Clinton earlier expressed her disappointment at the sentence.
She said the US would vigorously raise its concerns about the case with Tehran.
Washington, which has no diplomatic ties with Iran, was working with Swiss diplomats in Tehran to obtain details about the court's decision and ensure Ms Saberi's well-being, she said.
The verdict came despite calls by the Obama administration for Ms Saberi's release and diplomatic overtures to Iran after three decades of severed ties.
It raises deep suspicions over whether the case has been hijacked by hardliners within the Iranian government, eager to sabotage any reconciliation, the BBC's Jon Leyne reports from Tehran.
No details of the evidence against Ms Saberi have been made public.
Ms Saberi, who was arrested in January and went on trial this week, denies the charge and plans to go on hunger strike, her father said.
Reza Saberi told National Public Radio (NPR) that his daughter was tricked into making incriminating statements by officials who told her they would free her if she did.
Senators from Ms Saberi's home state of North Dakota described the court ruling as a shocking miscarriage of justice that would damage Iran's international credibility.
Ms Saberi has reported for a number of foreign news organisations including the BBC, NPR and Fox News.
"We are extremely concerned at the severe sentence passed on Roxana Saberi," the BBC Press Office said.
"Roxana's many friends in the BBC are saddened by the decision and are thinking of Roxana and her family at this difficult time."
NPR's chief executive, Vivian Schiller, said: "We are deeply distressed by this harsh and unwarranted sentence."
The conviction was also criticised by the New York-based Committee to Protect Journalists, which said her trial had "lacked transparency".
"We call on the Iranian authorities to release her on bail pending her appeal," said Mohamed Abdel Dayem, the group's Middle East and North Africa programme coordinator.
Public awareness of Ms Saberi's situation is low in Iran, where local media do not seem to have reported her arrest or trial in any way, our correspondent says.
The journalist originally faced the less serious accusation of buying alcohol, and later of working as a journalist without a valid press card.
Then, in a period of less than two weeks, the charge of spying was introduced, and she was tried by the Revolutionary Court and sentenced.
Ms Saberi's father (left) and lawyer say they are appealing
Ms Saberi's lawyer Abdolsamad Khorramshahi and her father confirmed that an appeal would be made.
A US-Iranian national, Ms Saberi has spent six years in Iran studying and writing a book.
The daughter of an Iranian father and a Japanese mother, she was once crowned Miss North Dakota and was among the top 10 finalists in Miss America 1998.
She holds two master's degrees, from Northwestern University in the US and from Cambridge University in the UK.