A plot to assassinate the Saudi ambassador to Washington DC involving elements of the Iranian regime has been foiled, the United States justice department has said.
US attorney general Eric Holder suggested at an official press conference on Tuesday that the plan had been conceived in Iran by the Quds force, part of Iran's Revolutionary Guard.
"The criminal complaint unsealed today exposes a deadly plot directed by factions of the Iranian government to assassinate a foreign ambassador on U.S. soil with explosives," he said.
"Through the diligent and coordinated efforts of our law enforcement and intelligence agencies, we were able to disrupt this plot before anyone was harmed. We will continue to investigate this matter vigorously and bring those who have violated any laws to justice."
Holder said two men had been charged in relation to the plot.
Meanwhile a worldwide travel warning has been issued for all US citizens with authorities citing terror concerns.
Manssor Arbabsiar, a naturalised U.S. citizen who holds an Iranian passport and was arrested last month in New York, is accused of working with members of an arm of the Iranian Revolutionary Guard Corps to devise an "international murder-for-hire scheme" targeting the Saudi Arabian ambassador to the United States, Adel Al-Jubeir.
The United States has accused Arbabsiar of orchestrating a $1.5m assassination plot with Gholam Shakuri, an Iranian-based member of the Qods Force, and other Iranian co-conspirators.
Holder said the conspiracy was "conceived, sponsored and directed from Iran" and constituted "a flagrant violation of U.S. and international law, including a convention that explicitly protects diplomats from being harmed".
He added: "The United States is committed to holding Iran accountable for its actions."
US secretary of state Hillary Clinton indicated the United States would seek to take action against Iran in response to the alleged plot.
"We are consulting with our friends and partners around the world about how we can send a very strong message that this kind of action, which violates intentional norms, must be ended and other areas where we can cooperate more closely in order to send as strong message to Iran and further measures to isolate it from international community will be considered," she said.
FBI Director Robert Mueller said the plot read "like the pages of a Hollywood script" and illustrated that "we live in a world where borders and boundaries are increasingly irrelevant".
Arbabsiar is said to have contacted a man in Mexico he believed to be a member of a Mexican drug gang who could carry out the assassination in Washington. The contact in fact worked for the US Drug Enforcement Agency.
A spokesman for Iranian president Mahmoud Ahmadinejad told CNN the United States had made up the plot to distract the American public from domestic issues.
"I think the US government is busy fabricating a new scenario and history has shown both the US government and the CIA have a lot of experience in fabricating these scenarios and this is just the latest one,” he said.
“I think their goal is to reach the American public. They want to take the public’s mind off the serious domestic problems they’re facing these days and scare them with fabricated problems outside the country.”
A spokesman for the US National Security Council said that the plot had first been brought to President Obama’s attention in June.
“The president was first briefed on this issue in June and directed his administration to provide all necessary support to this investigation,” he said.
Arbabsiar is reported to have been arrested in John F Kennedy airport in New York on 29 September and confessed to his involvement. Shakuri is said to be in Iran.
In a statement published on its website, the Saudi Embassy in Washington said the alleged plot was a "despicable violation" of international law.
"The Royal Embassy of Saudi Arabia would like to express its appreciation to the responsible agencies of the United States government for preventing a criminal act from taking place," it said.
"The attempted plot is a despicable violation of international norms, standards and conventions and is not in accord with the principles of humanity."
The British government has said it will support measures to hold Iran accountable for its actions if the allegations are proved to be true.
A Downing Street spokesman said: "We congratulate the US authorities on the successful operation to disrupt a conspiracy to attack diplomats in the United States.
"Indications that this plot was directed by elements of the Iranian regime are shocking. Directing an attack against another country is a flagrant violation of international law which calls for the strongest condemnation.
"The United Kingdom is in close touch with the US authorities on this case. We will support measures to hold Iran accountable for its actions."