A White House official described the assassination scheme as “extraordinarily brazen” and said it appeared to have been authorized by senior levels of the Quds Force, the elite covert-action arm of the Iranian Revolutionary Guards Corps. The force, headed by a fabled spymaster named Qassem Soleimani, is said to report directly to Iran’s Supreme Leader, Ayatollah Ali Khamenei.

“We see this as a chance to go out to capitals around the world and talk to allies and partners about what the Iranians tried to do,” the official said. “We’re not going to tolerate targeting a diplomat in Washington. We’re going to try to use this to isolate them to the maximum extent possible.”

Iran has denied the allegation and called it a “fabrication.” An assassination in Washington would be an unusually bold move by the Iranians. Their covert operatives bombed the U.S. embassy and Marine barracks in Lebanon in 1983, but they have never carried out a major attack inside the United States. If the plot alleged in 21-page indictment is true, it would be a sloppy one, with extremely poor tradecraft, by Quds Force standards.

The United States has been revving up the pressure campaign ever since one of the alleged plotters, Manssor Arbabsiar, was arrested Sept. 29 after a sting operation involving the Drug Enforcement Administration, two of whose Mexican drug-cartel informants Arbabsiar allegedly tried to hire as hit men. Arbabsiar began talking to U.S. authorities after his arrest, and at their direction discussed the plot on Oct. 4 with a Quds Force official in Iran named Gholam Shakuri, who was also indicted.

Immediately after the plot was broken up, national security adviser Tom Donilon traveled secretly to Saudi Arabia and briefed King Abdullah about it. He discussed with the Saudi monarch joint U.S.-Saudi efforts to contain the Iranians, including a major new sale of U.S. weapons.

The alleged Iranian plot is a potent piece of information partly because it surfaces at a time when Iran is facing serious problems, internally and externally. A domestic power struggle between President Mahmoud Ahmadinejad and Khamenei has paralyzed parts of the government. Meanwhile, Iran’s most important Arab ally, Syrian President Bashar al-Assad, is threatened by a growing domestic opposition.

Now added to this mix is an alleged Iranian plot to kill Amb. Adel Al-Jubeir, the prominent emissary of the leading Gulf Arab nation. It’s a case that, if the allegations true, illustrates the threat posed by Iran and, in that sense, will help galvanize international opposition to the regime.