A judge has sentenced a US-Iranian man to death for spying for the CIA, a report said on Monday, exacerbating Tehran-Washington tensions already high because of Western sanctions on the Islamic republic's nuclear programme.
Amir Mirzai Hekmati, a 28-year-old former Marine born in the United States, was "sentenced to death for cooperating with a hostile nation, membership of the CIA and trying to implicate Iran in terrorism," the Fars news agency said, quoting a verdict by the Revolutionary Court judge in Tehran.
Hekmati, born to an Iranian immigrant family living in the US, 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 Iranian 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 United States has demanded Hekmati's release.
The US state department said Iran did not permit diplomats from the Swiss embassy -- which handles Washington's interests in the absence of US-Iran ties -- to see Hekmati before or during his trial.
Hekmati's death sentence adds to a series of grave points of contention between Iran and the United States stemming from an escalating showdown over Tehran's nuclear programme.
Last month, Iran showed off what it said was a CIA drone it captured using cyberwarfare.
It also said Sunday it had arrested an unspecified number of "spies" who allegedly sought to carry out US plans to disrupt parliamentary elections in March.
The suspects were not identified by name or nationality and, as with numerous previous similar announcements, the accusation was not publicly substantiated.
Washington, for its part, said in October it thwarted a plot allegedly hatched in Tehran to assassinate the Saudi ambassador to the US. President Barack Obama last month also signed into law new sanctions targeting Iran's central bank.
The United States and its Western allies accuse Iran of seeking nuclear weapon capability.
Iran, though, insists its programme is exclusively for peaceful ends.
It 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 such a move would cross a "red line" and "we would take action and reopen the strait."
He told CBS television that while the United States does not believe Iran is actively developing an atomic bomb, it was "trying to develop a nuclear capability." "That's what concerns us. And our red line to Iran is do not develop a nuclear weapon. That's a red line for us," he said.