TEHRAN (AFP) - Iran on Tuesday put on trial an American man of Iranian descent accused of spying for the US Central Intelligence Agency, Fars news agency reported. "The first hearing in the trial of Amir Mirzai Hekmati, recently arrested for spying for the United States, started Tuesday" in a closed Tehran court, Fars reported. Hekmati, a 28-year-old former US Marine born in the United States to an Iranian immigrant family, was shown on Iranian state television mid-December saying in fluent Farsi and English that he was a CIA operative sent to infiltrate the Iranian intelligence ministry. The US government says Hekmati has been falsely accused and has demanded his immediate release.

It also said that the Swiss embassy, which represents US interests in Iran, had not been given access to him.

Hekmati's trial opened against a backdrop of heightened tensions between arch foes Washington and Tehran.

The United States is leading a Western push to ratchet up sanctions on Iran over its controversial nuclear programme.

Each side has accused the other of conducting clandestine operations.

The United States alleged in October that Tehran had a hand in a thwarted plot to assassinate the Saudi ambassador to Washington, while Iran this month showed off a CIA drone it said it captured through cyber warfare.

Fars said Hekmati's trial started with the prosecutor saying Hekmati was charged with cooperating "with the hostile US government and the US espionage services of the CIA."

It said Hekmati had admitted to trying to infiltrate Iran's intelligence services for the CIA, and quoted what it said was a confession by the Iranian-American.

Hekmati allegedly admitted to having a first interview with the CIA in 2009 and being trained for five months before being sent to Iraq, where he said he spent nine months.

After Iraq, the CIA hired him -- on a promised payment of $500,000 -- to infiltrate Iran, according to the presented confession.

"I was fooled by the US intelligence services. Even though I entered Iran with the mission of infiltrating the Iranian intelligence services to become a source of information for the CIA, I did not want to personally hurt Iran," Hekmati was quoted as saying, according to Fars.

"I had the intention of living in Iran and of not returning to the United States," he reportedly said.

Iran's intelligence ministry said Hekmati's cover was blown by Iranian agents who spotted him at the US-run Bagram base in Afghanistan.

The lawyer appointed to defend Hekmati criticised the accusation, Fars said, adding that the prosecutor rejected the lawyer's comments.

The judge handling the trial, Abolghasem Salavti, said he would deliver his verdict after hearing the defence lawyer's counter-argument.

US media quoted Hekmati's family as saying he had worked as an Arabic translator for the US Marines and had gone to Iran months ago to visit his Iranian grandmothers.

"My son is no spy," Hekmati's father, Ali Hekmati, a teacher in the state of Michigan, told the US television network ABC last week.

Espionage carries a potential death penalty in Iran.