An Iranian court has convicted an Iranian-American man of belonging to the CIA and sentenced him to death, a state radio report said.
The report on Monday escalated tensions between Iran and the US over the former's nuclear programme and its threat to close the crucial Strait of Hormuz oil transit route.
Iran says that Amir Merzayi Hekmati received special training and served at US military bases in Iraq and Afghanistan before heading to Iran under the cover of a family trip for his alleged intelligence mission.
The radio report on Monday did not say when the verdict was issued. Under Iranian law, Hekmati has 20 days to appeal.
The US denied Hekmati was a CIA spy, and criticised Iran for what it called a pattern of arresting innocent people for political reasons.
PJ Crowley, a former US state department spokesman, told Al Jazeera that an actual execution of Hekmati "would be very tragic and would really close the door on any normalised relations between the US and Iran".
"The crack is not wide as it is," he said. "We can never predict that it cannot get worse but it obviously can."
Hekmati, a 28-year-old former military translator, was born in Arizona and graduated from high school in Michigan. His family is of Iranian origin.
His father, a professor at a community college in Flint, Michigan, has said his son is not a CIA spy and was visiting his grandmothers in Iran when he was arrested.
The Iranian court convicted Hekmati of working with a hostile country, belonging to the CIA and trying to accuse Iran of involvement in terrorism, Monday's radio report said.
Hekmati is a dual US-Iranian national, but Iran considers him merely Iranian since the country's law does not recognise dual citizenship.
In its ruling, a branch of the Tehran Revolutionary Court described Hekmati as a mohareb, an Islamic term that means a fighter against God, and a mofsed, or one who spreads corruption on earth. Both terms appear frequently in Iranian court rulings.
In a closed court hearing in late December, the prosecution asked for the death penalty for Hekmati.
The US has called on Iranian authorities to grant Swiss diplomats access to him in prison. The Swiss government represents US interests in Iran because the two countries do not have diplomatic relations.
The Marine Corps said "Amir Nema Hekmati" served between 2001 and 2005, including one deployment to Iraq in 2004 and a stint at the military language institute in Monterey, California.
The Marine records do not indicate any deployment to Afghanistan. It was not clear why the middle name was listed differently.
On December 18, Iran's state TV broadcast video of Hekmati delivering a purported confession.
In a statement released the same day, Iran's intelligence ministry said its agents had identified Hekmati at Bagram air field in neighbouring Afghanistan. Bagram is the main base for American and other international forces outside Kabul, the Afghan capital.
It is not clear exactly when he was arrested, although Iranian news reports have said he was detained in late August or early September.
At the time of his trip to Iran, Hekmati had been working in Qatar as a contractor for a company "that served the marines", his father said, without providing specific details.
The Iranian court's decision came as the US gave warning that it would consider reports of Iran's enriching uranium to 20 per cent a "further escalation" of its pattern of violating obligations under UN Security Council resolutions.
The International Atomic Energy Agency (IAEA), the UN nuclear watchdog, confirmed on Monday that Iran had started enriching uranium up to 20 per cent at an underground facility at Fordo near the city of Qom, and that all atomic material there was under its surveillance.
Victoria Nuland, the US state department spokesperson, also repeated the US appeal to Latin American countries not to have close relations with Iran and impress upon it that its course in the nuclear dialogue "is the wrong one."
The appeal was made as Mahmoud Ahmadinejad, the Iranian president, met Hugo Chavez, the Venezuelan president, in Caracas at the start of a tour of Latin America that is also set to take him to Nicaragua, Cuba and Ecuador.
"We think it's in the interest of all countries, including the countries that he's visiting in Latin America, that Iran proves the peaceful intent of its nuclear programme to the world," Nuland said.
Ahmadinejad's travels in Latin America are seen as an effort to side-step growing international tensions and find new markets as the European Union considers an outright embargo on Iranian oil. The US could impose sanctions on foreign countries doing business with its central bank, which would also seriously block the inflow of money for its major export, oil.