US authorities said they had broken up a plot by two men linked to Iran's security agencies to assassinate Saudi Ambassador Adel al-Jubeir. One was arrested last month while the other was believed to be in Iran.

Iran denied the charges. But President Barack Obama called the plot a "flagrant violation of US and international law" and Saudi Arabia said it was "despicable."

Revelation of the alleged plot, and the apparent direct ties to the Tehran government, had the potential to further inflame tensions in the Middle East, and the United States said Tehran must be held top account.

Secretary of State Hillary Clinton, in an interview, expressed hope that countries that have hesitated to enforce existing sanctions on Iran would now "go the extra mile."

At a news conference, FBI Director Robert Mueller said the convoluted plot, involving monitored international calls, Mexican drug money and an attempt to blow up the ambassador in a Washington restaurant, could have been straight from a Hollywood movie.

US Attorney General Eric Holder alleged that the plot was the work of the Islamic Revolutionary Guard Corps, which is the guardian of Iran's 32-year-old revolution, and the Quds force, its covert, operational arm.

US officials identified the two alleged plotters as Gholam Shakuri, who is a member of the Quds force, and Manssor Arbabsiar, who was arrested on September 29 when he arrived at John F. Kennedy International Airport from Mexico.

Arbabsiar, 56, who is a naturalized US citizen and holds an Iranian passport, initially cooperated with authorities after being arrested at John F. Kennedy International Airport in New York on September 29.

He made calls to Shakuri after being arrested and acted as if the plot was still a go, court documents said.