The United States has condemned the death sentence handed to US-Iranian ex-Marine Amir Mirzai Hekmati in Iran and says allegations he worked for the CIA are "false".
"We have seen Iranian press reports that Mr Hekmati has been sentenced to death by an Iranian court. If true, we strongly condemn such a verdict and will work with our partners to convey our condemnation to the Iranian government," National Security Council spokesman Tommy Vietor said on Monday.
Hekmati, a 28-year-old former US Marine born in the United States to an Iranian family, was "sentenced to death for co-operating with a hostile nation, membership of the CIA and trying to implicate Iran in terrorism", a judge in Tehran ruled, according to reports by the Fars and ISNA news agencies.
The ruling further ratcheted up a war of words between Iran and the United States as tensions flare over Tehran's nuclear program and an Iranian threat to close the crucial Gulf of Hormuz oil transit route.
"The Iranian regime has a history of falsely accusing people of being spies, of eliciting forced confessions, and of holding innocent Americans for political reasons," Vietor said.
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He called on Iran to offer immediate access for Swiss diplomats who take care of American consular issues in Iran to Hekmati, and for him to be provided with legal counsel and released.
Hekmati has 20 days to appeal the verdict, ISNA quoted chief prosecutor Gholam Hossein Mohseni Ejei as saying, without specifying when the sentence was handed down.
The accused was shown on state television in mid-December saying in fluent Farsi and English that he was a Central Intelligence Agency operative sent to infiltrate Iran's intelligence ministry.
Iranian officials said his cover was blown by agents for Iran who spotted him at the US-run Bagram military air base in Afghanistan.
But Hekmati's family in the United States told US media he had travelled to Iran to visit his grandmothers and insisted he was not a spy.
In his sole trial hearing, on December 27, prosecutors relied on Hekmati's "confession" to say he tried to penetrate the intelligence ministry by posing as a disaffected former US soldier with classified information to give.
The death sentence comes after the case of three other Americans who were held in Iran on spying charges after hiking in 2009 along the unmarked Iran-Iraq border. All three were eventually released, one in 2010 and the other two in September 2011, despite being sentenced to eight years in prison.
