WASHINGTON – Two men, including a naturalized U.S. citizen from Iran, were charged Tuesday in a terrorist plot "conceived, supported and directed" by elements of the Iranian government to assassinate the Saudi Arabian ambassador to the United States while he was at a restaurant in the nation's capital, Attorney General Eric Holder said Tuesday.

He said the plot was foiled by U.S. drug agents in Mexico and a paid informant posing as a willing co-conspirator, recruited from the ranks of a drug-trafficking cartel.

Holder said Ambassador Adel Al-Jubeir was never in danger because the plot was monitored by U.S. undercover agents from the time it was outlined at a May meeting in Mexico between an informant working for the Drug Enforcement Administration and the U.S. suspect. The attack, he said, was to be the first salvo in a series of assaults.

Secretary of State Hillary Rodham Clinton said the plot "crosses a line" in Iran's state sponsorship of terrorism and will further isolate the Islamic republic. Clinton said she and President Obama were reaching out to other countries to work against what is becoming a "clearer and clearer threat" from Iran.

The Saudi Embassy in Washington called the suspected assassination plot a "despicable violation of international norms, standards and conventions and is not in accord with the principles of humanity." The Saudi release made no mention of the Iranian government.

Adel Al-Jubeir, Saudi Ambassador to the U.S. 2004 USA TODAY photo

Iran denied the allegations, saying the U.S. government was "fabricating problems" to take the focus off America's domestic woes.

Manssor Arbabsiar, 56, described in court documents as a U.S. citizen holding both Iranian and U.S. passports, was charged with Gholam Shakuri, an Iran-based member of the country's Quds Force, a special-operations unit of the Islamic Revolutionary Guard Corps, a paramilitary arm of the Iranian government.

Both were charged with conspiracies to murder a foreign official, to use a weapon of mass destruction, to commit an act of international terrorism and to travel internationally as part of a murder-for-hire plot.

Holder said Arbabsiar, who has a residence in Round Rock, Texas, was arrested last month at John F. Kennedy International Airport in New York after he was refused entry into Mexico, where authorities say he was scheduled to finalize the assault plan with the U.S. informant.

Arbabsiar confessed to U.S. investigators, saying he was "recruited, funded and directed by men he understood to be senior officials in Iran's Quds Force," according to court documents.

In a worldwide travel alert issued late Tuesday, the State Department warned Americans abroad to be aware of possible terror attacks.