"This plot, very fortunately disrupted by the excellent work of our law enforcement and intelligence professionals, was a flagrant violation of international and U.S. law and a dangerous escalation of the Iranian government's longstanding use of political violence and sponsorship of terrorism," Clinton told a think tank audience in Washington.

Clinton said Iran's alleged part in the conspiracy, which was revealed by U.S. law enforcement officials on Tuesday, showed Tehran was willing to flout established international conventions on protecting diplomats that it itself had signed.

"This kind of reckless act undermines international norms and the international system. Iran must be held accountable for its actions," Clinton said, noting that the United States had already imposed targeted sanctions on "individuals within the Iranian government who are associated with this plot and Iran's support for terrorism."

"We will work closely with our international partners to increase Iran's isolation and the pressure on its government and we call upon other nations to join us in condemning this threat to international peace and security," she said.

Saudi Arabia said on Wednesday Iran would "pay the price" for the alleged plot, while Tehran called the accusation a fabrication designed to sow discord in the region already on edge over Iran's nuclear ambitions.

(Reporting by Andrew Quinn; Editing by Jackie Frank)