The plot included the assassination of the Saudi Arabian ambassador to the U.S., Adel al-Jubeir, and subsequent bomb attacks on the Saudi and Israeli embassies in Washington, D.C.
U.S. officials said bombings of the Saudi and Israeli embassies in Buenos Aires, Argentina, were also discussed.
Two people, identified as Manssor Arbabsiar and Gholam Shakuri, have been charged in the apparent murder-for-hire scheme.
Arbabsiar, 56, was arrested Sept. 29. He appeared in court Tuesday in New York to face charges including conspiracy to murder a foreign official, conspiracy to use a weapon of mass destruction and conspiracy to commit an act of international terrorism. He did not enter a plea.
"Arbabsiar, a naturalized United States 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," U.S. Atty. Gen. Eric Holder said.
Shakuri, an Iranian citizen, remains at large.
Holder said during a news conference that the plot was directed by senior elements of Iran's government.
The U.S. said the $1.5 million plot was discovered when a DEA informant with a high rank in a Mexican drug cartel was approached by the conspirators to kill the Saudi ambassador.
Mexican officials then worked with the U.S. on the arrest.
"The U.S. is committed to holding Iran accountable for its actions," Holder said.
The Iranian government denies any knowledge of the plot, calling it a fabrication being made up by U.S. officials trying to divert attention from domestic problems.
Officials say President Barack Obama was briefed on the situation and ordered all necessary support to be provided for the investigation.
"The disruption of this plot is a significant achievement by our intelligence and law enforcement agencies, and the president is enormously grateful for their exceptional work in this instance and countless others," said Tommy Vietor, National Security Council spokesman.