* Death sentence likely to worsen U.S.-Iran tensions
* Americans deny Hekmati a spy, parents plea for mercy
* Friction increasing over Iran's nuclear program (Adds White House comment)
By Parisa Hafezi
TEHRAN, Jan 9 (Reuters) - Iran's Revolutionary Court has sentenced an Iranian-American man to death for spying for the CIA, officials said on Monday, a move likely to aggravate U.S.-Iranian tensions already high because of Tehran's disputed nuclear program.
The United States denies that Amir Mirza Hekmati is a spy and has demanded his immediate release. The White House said it was trying to verify the report on his sentencing.
"If true, we strongly condemn such a verdict and will work with our partners to convey our condemnation to the Iranian government," said Tommy Vietor, spokesman for the White House National Security Council.
Western nations have recently expanded punitive economic sanctions against Iran over suspicions it is trying to develop atomic bombs under the cover of a civilian atomic energy program. The Islamic Republic denies this.
But word from diplomatic sources in Vienna, headquarters of the U.N. nuclear watchdog, that Iran has begun enriching uranium in a mountain bunker protected from possible Western air strikes is likely to heat the atmosphere further. [ID:nL6E8C91N4 ]
"Amir Mirza Hekmati was sentenced to death...for cooperating with the hostile country America and spying for the CIA (Central Intelligence Agency)," the student news agency ISNA quoted judiciary spokesman Gholamhossein Mohseni-Ejei as saying.
"The court found him Corrupt on the Earth and Mohareb (one who wages war on God). Hekmati can appeal to the Supreme Court."
Iran's highest court must confirm all death sentences. When it will rule in Hekmati's case was not known.
Hekmati, a 28-year-old of Iranian descent born in the southwestern U.S. state of Arizona, was arrested in December and Iran's Intelligence Ministry accused him of receiving training at U.S. bases in neighboring Afghanistan and Iraq.
"Allegations that Mr. Hekmati either worked for, or was sent to Iran by the CIA are false," Vietor said. "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."
The United States urged Iran to grant Hekmati access to legal counsel and to "release him without delay."
Iran's judiciary said Hekmati admitted to having links with the CIA but denied any intention of harming Iran, which has had no relations with the United States since its 1979 Islamic Revolution. Mutual antagonism has reigned since.
Hekmati's parents issued a statement, signed by his mother, Behnaz Hekmati, denying their son was a spy and asking for mercy for him. They said the former U.S. military translator was visiting relatives in Iran for the first time when he was arrested.
"My husband Ali and I are shocked and terrified by the news that our son, Amir, has been sentenced to death," it said.
"Amir did not engage in any acts of spying, or 'fighting against God,' as the convicting Judge has claimed in his sentence. Amir is not a criminal. His very life is being exploited for political gain."
The State Department has said Iran did not permit diplomats from the Swiss Embassy, which represents U.S. interests in Iran, to see him before or during his trial.
Hekmati graduated from a Michigan high school. His father Ali is a professor at a community college in Flint, Michigan.
SPY NETWORK UNCOVERED
Hekmati, whose trial ended on Jan. 2, was shown on Iranian state television in December saying he was a CIA operative sent to infiltrate the Iranian intelligence ministry.
Iran also said on Monday it had broken up an alleged U.S.-linked spy network that planned to "fuel unrest" before the March parliamentary election, the first nationwide vote since the country's 2009 disputed presidential vote.
"The detained spies were in contact with foreign countries through cyberspace," Intelligence Minister Haydar Moslehi was quoted by state television as saying. He gave no information about the nationalities and the number of those detained.
Iran, which often accuses its foes of trying to destabilize its Islamic system, said in May it had arrested 30 people on suspicion of spying for the United States and later 15 people were indicted for spying for Washington and Israel.
Despite mounting international pressure and sharpened rhetoric, Iran seems determined to stick to its nuclear course ahead of the parliamentary election, to be followed by a presidential ballot in 2013.
The United States is leading efforts to tighten sanctions on Iran. Washington and Israel say they do not rule out carrying out pre-emptive military strikes on Iran's nuclear sites if diplomacy fails to resolve the row.
Iran has threatened to close Gulf oil shipping lanes through the Strait of Hormuz that are vital to the global economy if the West carries out plans to bar Iranian crude exports, or if Iranian nuclear sites came under military attack. (Additional reporting by Hossein Jaseb, Mitra Amiri and Ramin Mostafavi in Tehran, Caren Bohan in Washington, Andrew Stern in Chicago; Writing by Parisa Hafezi; Editing by Mark Heinrich and Doina Chiacu)