The letter said that "The Iranian nation seeks a world free from terrorism and considers the current U.S. warmongering and propaganda machine (working) against Iran as a threat not just against Iran but to the peace and stability in the Persian Gulf region," according to Press TV.

Saudi Ambassador to the United States Adel A Al-Jubeir

Iran's Foreign Ministry summoned Swiss charge d'affaires on Wednesday to protest against Washington's accusations.

The chief of Iranian Foreign Ministry's North America office told the Swiss charge d'affaires that Iran condemns the "worthless claims of the U.S. government and warns against such political propaganda."

Over the past three decades "the Islamic Republic has been the victim of terrorist acts of the terrorist groups which were supported by the United States," the official was quoted by IRIB as saying.

The Swiss charge d'affaires said Iran's protest will be transferred to the U.S. government and their response will be returned to Iran.

The Swiss embassy in Tehran represents the U.S. interests in Iran since Tehran and Washington severed diplomatic ties in 1980.

On Tuesday, Iran ambassador to the UN handed a letter of complaint to the UN Secretary-General Ban Ki-moon, voicing outrage over the U.S. allegations against his country.

The U.S. State Secretary Hillary Clinton pledged on Wednesday that "Iran must be held accountable for its actions."

She said that the plot 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."

"This kind of reckless act undermines international norms and the international system," Clinton added.

Also, U.S. authorities issued a statement, saying that "The Department of State alerts U.S. citizens of the potential for anti-U.S. actions following the disruption of a plot, linked to Iran, to commit a significant terrorist act in the United States."

The Saudi embassy in Washington on Tuesday thanked the United States for preventing the alleged assassination attempt on the Saudi ambassador.

"Plot to assassinate ambassador is a despicable violation of international norms, standards, conventions, not in accord with principles of humanity," the embassy said on its Tweeter account.

Saudi prince Turki al-Faisal said in London "The burden of proof is overwhelming ... and clearly shows official Iranian responsibility for this. Somebody in Iran will have to pay the price."