An Iranian judge sentenced a US-Iranian man to death for spying for the CIA, media reported Monday, exacerbating high tensions in the face of Western sanctions on the Islamic republic's nuclear programme.
Amir Mirzai Hekmati, a 28-year-old former Marine born in the United States to an Iranian family, was "sentenced to death for cooperating with a hostile nation, membership of the CIA and trying to implicate Iran in terrorism," the judge in Tehran ruled, the Fars and ISNA news agencies reported.
The United States condemned the reported death sentence and said allegations that Hekmati worked for the CIA were "false."
Amnesty International urged Iran not to execute Hekmati after an "unfair trial."
Hekmati has 20 days to appeal, 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.
He had been arrested months earlier.
Iranian officials said his cover was blown by agents for Iran who spotted him at the US-run Bagram military air base in neighbouring Afghanistan.
But Hekmati's family in the United States told US media he had travelled to Iran to visit his grandmothers and 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.
Hekmati was tried as an Iranian citizen, not a US one, because Iran does not recognise dual nationality.
The United States condemned the sentence and called on Iran to offer immediate access for Swiss diplomats who take care of US consular issues in Iran to Hekmati, and for him to be provided with legal counsel and released.
"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," said Tommy Vietor, spokesman for the US' National Security Council.
Amnesty also questioned the timing of the reported decision to execute Hekmati.
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"We know from past experience that the Iranian authorities sometimes rush forward with executions of political prisoners -- including dual nationals -- at politically sensitive times and we fear that this execution could happen within days or weeks," said the rights group's interim director for the Middle East and North Africa, Ann Harrison.
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 released, one in 2010 and the other two in September 2011, despite being sentenced to eight years in prison.
Hekmati's case, though, comes as the temperature rises between Tehran and Washington over US-led moves to hit Iran's oil exports as part of new sanctions to be imposed over its controversial nuclear programme.
Iran insists its programme is exclusively for peaceful ends.
But the UN atomic agency in Vienna confirmed on Monday that Tehran is now enriching uranium at a new site in a hard-to-bomb mountain bunker, in a move set to stoke Western suspicions further that the Islamic Republic is seeking nuclear weapons capability.
"The IAEA can confirm that Iran has started the production of uranium enriched up to 20 percent... in the Fordo Fuel Enrichment Plant," International Atomic Energy Agency spokeswoman Gill Tudor said in a statement.
A Western diplomat at the IAEA headquarters called the development of the Fordo facility, 150 kilometres (90 miles) southwest of Tehran, a "provocation" that served "only to increase the international community's serious concerns."
Iran's envoy to the IAEA, Ali Asghar Soltanieh, ealier told Iran's Al-Alam Arabic broadcaster that all his country's nuclear activities, "notably uranium enrichment at Natanz and Fordo, are under the supervision of the International Atomic Energy Agency."
US-Iranian tensions worsened last month when Iran showed off what it said was a CIA drone it captured using cyberwarfare.
Washington, for its part, said in October it thwarted a plot allegedly hatched in Tehran to assassinate the Saudi ambassador. President Barack Obama last month also signed into law new sanctions targeting Iran's central bank.
The country's supreme leader, Ayatollah Ali Khamenei, on Monday dismissed claims that "sanctions and pressures" would prevent Tehran from pursuing its goals.
Iran has threatened to close the strategic Strait of Hormuz at the entrance to the Gulf -- a chokepoint for 20 percent of the world's oil -- if it is threatened militarily or by sanctions.
US Defence Secretary Leon Panetta warned on Sunday that any such move would cross a "red line" and "we would take action and reopen the strait."