WASHINGTON — The United States on Tuesday dismissed as "a rant" an Iranian letter to Washington over allegations of an Iranian plot to murder the Saudi ambassador in Washington.

"We did receive a lengthy diplomatic note from the Swiss protecting power on behalf of the Iranians," US State Department spokeswoman Victoria Nuland said after Washington received the letter from Swiss intermediaries.

In the absence of US-Iranian diplomatic relations for more than three decades, Switzerland acts on behalf of US interests in Tehran.

"It was about seven pages. It was a rant. It was full of all kinds of denials. There was not a lot new in there from our perspective," Nuland told reporters.

In Tehran, Iran's foreign ministry spokesman Ramin Mehmanparast said Tuesday that Iran had sent a letter to the United States seeking an "official apology of the Americans in protest of this made-up scenario."

He explained the Iranian letter was in response to a US one, whose contents he did not divulge.

On October 11, US officials alleged Iran's Quds Force -- an elite wing of the Revolutionary Guards -- plotted to kill the Saudi ambassador to Washington by hiring assassins from a Mexican drug cartel for $1.5 million.

An Iranian-American car salesman accused of involvement in the plot pleaded not guilty in a New York court last week.