Clinton and other U.S. officials urged the rest of the world to join Washington in condemning the scheme, which she said violated U.S. and international law as well as Iran's treaty obligations to protect diplomats.
"This kind of reckless act undermines international norms and the international system. Iran must be held accountable for its actions," Clinton said.
Her remarks at a Washington conference were part of an Obama administration campaign to use the alleged plot as a springboard for increased international condemnation of Iran and perhaps for new sanctions.
"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," Clinton said.
Her words strongly suggested that the U.S. wants some new action against Iran from the U.N. Security Council, which has already approved several rounds of mild to moderate sanctions on Iran over its disputed nuclear program.
The State Department sent a cable to all American embassies and consulates around the world telling them to put the Iran case before their host governments. Officials said the cable, sent late Tuesday by Deputy Secretary of State William Burns and classified "secret," tells them to detail the evidence against Iran as presented by federal prosecutors.
Burns also met Wednesday with the entire Washington-based diplomatic corps at the State Department, while U.S. Ambassador to the United Nations Susan Rice and other officials were briefing members of the U.N. Security Council on the foiled plot, State Department spokeswoman Victoria Nuland said.
U.S. officials said Rice was not asking for any specific action now, but that the U.S. probably would make such a request soon. Officials spoke on condition of anonymity because they were not authorized to publicly discuss the closed-door diplomacy or the classified cable.
"We are looking for countries to join us in increasing the political and the economic pressure on Iran," Nuland told reporters. On the political side, she said the U.S. wanted firm condemnations of the plot. On the economic side, she said the U.S. wanted countries to redouble their efforts to enforce U.N. sanctions, national sanctions, regional sanctions, and to impose additional sanctions.
That would "send the message that this is not the kind of behavior that can be tolerated in the international community," she said.
Prosecutors on Tuesday accused Iran of plotting to hire a Mexican drug cartel to kill the Saudi envoy with a bomb attack in Washington. President Barack Obama called it "a flagrant violation of U.S. and international law." Iran has denied the charges.
The officials said the cable instructs diplomats to ask nations to consider appropriate steps against Iran in response to the alleged scheme. The cable does not suggest any specific measures, the officials said.
Obama's top national security aides have said the administration will lobby for the imposition of new international sanctions as well as for individual nations to expand their own penalties against Iran.
Vice President Joe Biden said in a television interview Wednesday that "it's critically important that we unite the world in the isolation" of Tehran and that "whatever action is ultimately taken ... that it's not the United States versus Iran."
He called the purported assassination plot "really over the top."
At a conference in London, a former Saudi ambassador to Washington, Prince Turki al-Faisal, said Iran "will have to pay the price" for the plot.
"The burden of proof and the amount of evidence in the case is overwhelming and clearly shows official Iranian responsibility for it," he said. "This is unacceptable. Somebody in Iran will have to pay the price, and that price will have to be on the terms acceptable to the norms and practices in Iran and other countries."
Britain's government said Wednesday it was consulting with the U.S. and others over new international sanctions against Iran. "We would support any measures that help hold Iran accountable for its actions," said Steve Field, spokesman for British Prime Minister David Cameron.
The French foreign ministry said it had been briefed and considered the issue "an extremely serious matter, a scandalous violation of international law."
Clinton and other U.S. officials said the alleged plot is further proof that Iran is the world's leading state sponsor of terrorism, a label Washington has for decades applied to the Iranian government. The officials said it also underscores concerns that despite its denials Iran is trying to develop nuclear weapons under cover of a civilian atomic energy program.
The purported plan was to carry out the assassination with a bomb attack while Al-Jubeir dined at his favorite restaurant.
Iran's parliament speaker, Ali Larijani, dismissed the U.S. charges as a "childish game."
"We have normal relations with the Saudis," Larijani added. "There is no reason for Iran to carry out such childish acts."
In New York, Alireza Miryousefi, head of the press office of the Iranian mission to the United Nations, sent Secretary-General Ban Ki-moon a letter "to express our outrage" over the allegations.
"The U.S. allegation is, obviously, a politically-motivated move and a showcase of its long-standing animosity toward the Iranian nation," the letter said.
The State Department late Tuesday warned Americans around the world of the potential for terrorist attacks against U.S. interests. It said Iranian-sponsored attacks could include strikes in the United States.
Saudi Arabia is the main Sunni Muslim power center in the Middle East, and the one most closely allied with the United States, Iran's declared enemy. Iran is the most powerful and influential Shiite Muslim state. The two have long vied for power and influence across the region. Saudi Arabia and other countries like Bahrain have accused Iran of trying to create dissent in their countries this year, during democracy movements across the region.
The U.S. criminal complaint said the Iranian plotters hired a would-be assassin in Mexico who was a paid informant for the U.S. Drug Enforcement Administration and told U.S. authorities all about their plot, which they code-named "Chevrolet."
Manssor Arbabsiar, a 56-year-old U.S. citizen who also holds an Iranian passport, was charged along with Gholam Shakuri, who authorities said was a Quds Force member and is still at large in Iran. The Treasury Department listed addresses for Arbabsiar in two Texas cities -- the Austin suburb of Round Rock and the Gulf city of Corpus Christi -- and prosecutors say he frequently traveled to Mexico for business.
The Obama administration has often said that no option is off the table with Iran, a position that a U.S. official said had not changed Tuesday. But the official, who spoke on condition of anonymity because he was not authorized to discuss the policy publicly, said the emphasis now is on increasing diplomatic and economic pressure on Iran.