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 programme.

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 programme. 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.

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 neighbouring Afghanistan and Iraq.

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 family said earlier in a statement after his arrest that the former U.S. military translator was visiting his grandmothers in Iran when he was arrested.

We have struggled to provide Amir with an attorney in Iran. We have sought to hire at least 10 different attorneys to no avail, the family's statement said. It said Hekmati's only advocate in Iran was a government-appointed lawyer who he met on the first day of his trial.

The U.S. government has demanded Hekmati's release, saying he has been falsely accused. State Department spokeswoman Victoria Nuland said last month that Iran had not permitted diplomats from the Swiss Embassy, which represents American 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 January 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 ahead of 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 destabilise 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.

Supreme Leader Ayatollah Ali Khamenei said on Monday that Iran would not yield to the pressure of sanctions imposed by the West to get the Islamic Republic to change its nuclear course.

The June 2009 presidential election was followed by eight months of anti-government street protests. Iranian authorities accused Washington and its allies of supporting the opposition unrest to overthrow Tehran's clerical establishment.

The opposition rejected the 2009 vote, saying it was rigged to secure hardline Mahmoud Ahmadinejad's re-election.

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's refusal to halt nuclear enrichment - which can have both military and civilian purposes - has drawn four rounds of U.N. sanctions since 2006 and separate U.S. and European steps.

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 mooted 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, Christopher Wilson in Washington, Fredrik Dahl in Vienna; Writing by Parisa Hafezi; Editing by Mark Heinrich)