Iran's supreme leader, Ayatollah Ali Khamenei, warned on Sunday of a "decisive response" to any "inappropriate action" over US claims of a Tehran-directed assassination plot.
"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."
The remarks came as US officials consult with allies and other countries on ratcheting up pressure on Iran, which is already subject to UN and US sanctions over its controversial nuclear programme.
Iran has fiercely denied any involvement in the thwarted plot to assassinate the Saudi ambassador to Washington.
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.
Saudi Arabia has also said it will make "a suitable response" over the alleged plot, which it has requested be brought to the attention of the UN Security Council.
Britain and France have said they will back any measure to punish Iran on the issue.
The US Justice Department and FBI say the alleged plot 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.
The money was allegedly a down payment for a $1.5-million-dollar hit on the Saudi ambassador, Adel al-Jubeir, possibly through the bombing of a Washington restaurant.
An Iranian used-car salesman who is a naturalised US citizen living in Texas, Manssor Arbabsiar, 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.
Arbabsiar is in US custody, charged with the plot.
The US Justice Department has also charged another Iranian identified as Gholam Shakuri, who is said to be an Iran-based Quds Force operative who flew to the United States to speak with Ababsiar. Shakuri is now believed to be in Iran.