(CNN) -- Iran's supreme leader took verbal jabs at the United States Saturday in his first public reaction since the United States accused Iran of plotting to assassinate the Saudi ambassador to Washington.
Ayatollah Ali Khamenei labeled the allegations "meaningless and absurd."
"They (the U.S.) want to isolate Iran," Khamenei said over chants of "down with America" in a speech before thousands in the western Iranian city of Gilangharb.
Also, an Iranian official said claims by the United States that a high-level U.S. diplomat had met Wednesday with an Iranian counterpart over the plot were untrue.
"There were no kinds of negotiations between the two countries, and there was not such a contact," said Alireza Miryousefi, press secretary for the Iranian Mission to the United Nations.
The United States had reported having had "direct contact with Iran" about the alleged plot.
State Department spokeswoman Victoria Nuland disclosed the contact to reporters. A senior administration official told CNN it occurred Wednesday and was initiated by the United States.
Two State Department officials said U.S. Ambassador to the United Nations Susan Rice met with Mohammad Khazai, Iran's permanent representative to the United Nations.
Ahead of Saturday's rebuttal of the claims of diplomatic contact, Iranian officials had previously declined to confirm the meeting.
U.S. authorities have accused Iran of being involved in a plot to kill the Saudi Arabian ambassador to the United States, Adel Al-Jubeir, in spring 2012.
The alleged scheme involved a connection to the Quds Force, a branch of the Islamic Revolutionary Guard, which formally answers to Khamenei.
Manssor Arbabsiar, a 56-year-old naturalized U.S. citizen, and Gholam Shakuri, an Iran-based member of Iran's Islamic Revolutionary Guard Corps, are accused of conspiring to hire hit men from a Mexican drug cartel to bomb a restaurant, where the ambassador would have been.
Authorities developed the case against the suspects with the help of an undercover informant posing as an associate of a Mexican drug cartel, according to officials and an FBI agent's affidavit.
Reza Aslan, a religious scholar and author, told CNN on Saturday that the described plot "just does not fit the Quds Force's M.O. (modus operandi)."
Using a drug cartel would be risky and a Quds Force agent would be more reliable than Arbabsiar, a used-car salesman in Texas, he said.
"It's sloppy. It's uncharacteristic," said Aslan. "It really does not serve Iran's interest in any legitimate way."
Iran could more easily target Saudi diplomats in the Middle East, Africa and elsewhere, Aslan said. "Doing so on U.S. soil is unmistakably an attack on the United States, not on Saudi Arabia."
CNN's Mitra Mobasherat and Hala Gorani contributed to this report