Iran on Sunday demanded a "consular visit" to Iranian-US national Manssor Arbabsiar, who is in United States custody facing charges over an alleged Tehran-directed assassination plot, Fars news agency said.
It said a letter to the US authorities demanding such access had been sent via the Swiss embassy in Tehran, which looks after US interests there as Iran and the United States have no diplomatic ties.
"There is no doubt the American accusations are without basis but a handover of personal information of the accused and a consular visit are an obligation for the US government," an Iranian diplomat told the Swiss charge d'affaires at a meeting at the foreign ministry, Fars said.
"Any delay in this regard is against international rules and the responsibilities of the American government," he added.
The US Justice Department and FBI say the alleged plot to kill Saudi Arabia's ambassador to the US leads back to officials inside the Quds Force, a special operations outfit within Iran's elite Revolutionary Guards.
Quds Force personnel are said to have transferred nearly $100,000 to the bank account used by a member of the Mexican drug cartel who was really a paid US informant.
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The money was allegedly a down payment for a $1.5-million-dollar hit on the Saudi ambassador to the US, Adel al-Jubeir, possibly through the bombing of a Washington restaurant.
Arbabsiar, an Iranian used-car salesman who is a naturalised US citizen living in Texas, is said to have confessed to acting as the go-between for his cousin, whom he described as a high-ranking official in the Quds Force, and the Mexican cartel.
Earlier Sunday, Iran's supreme leader, Ayatollah Ali Khamenei, warned of a "decisive response" to any "inappropriate action" over the US allegations.
"If American officials are entertaining any illusions, they should know that any inappropriate action -- whether political or security-related -- will meet with the Iranian people's decisive response," he said in a speech in western Iran, according to his official website.
Khamenei, who has already labelled the plot claims "absurd," warned: "The Islamic republic of Iran will face off any plot, or destructive or obstructive measures, with all its might."
Iran has fiercely denied any involvement in the thwarted plot.
But US President Barack Obama has vowed Iran will "pay a price" for what he says is incontrovertible proof it had a hand in trying to contract a Mexican drug cartel to carry out the hit.