"Iran is a civilized nation and doesn't need to resort to assassination" Ahmadinejad was quoted by the official IRNA news agency as saying Sunday. "Terror belongs to you," he said, addressing the United States.
Two men, including a member of Iran's special foreign actions unit known as the Quds Force, have been charged in New York federal court with conspiring to kill the Saudi diplomat, Adel Al-Jubeir.
Iranian officials have consistently denied the allegations since they first emerged last week.
Ahmadinejad's statements, and similar Saturday remarks by Supreme Leader Ayatollah Ali Khamenei, are the first comments made by the country's two highest leaders.
Khamenei, who has the final say on all state matters in Iran, had said that the U.S. seeks to bring greater pressure on Iran through making unsubstantiated allegations.
"They (Americans) conduct such conspiracies against us frequently, which are all useless and ineffective. They say that they want to isolate Iran but they have become isolated themselves," Khamenei said on state TV.
"By attributing an absurd and meaningless accusation to a few Iranians, they tried ... to show that Iran is a supporter of terrorism ... This conspiracy didn't work and won't work," he said.
In a formal statement released Saturday, the Iranian government said it has no connection to Manssor Arbabsiar, the man arrested in the alleged plot.
"Unilaterally announcing accusations without showing documentation and creating a media wave against Iran is in no way compatible with legal logic, and can only be a purely media and political show," it said.
President Barack Obama said Thursday that the U.S. will be able to support all of its allegations that Iran was directly involved in a plot to kill Al-Jubeir.
Arbabsiar is a 56-year-old naturalized U.S. citizen who also had an Iranian passport. In May 2011, the criminal complaint says, he approached someone he believed to be a member of the vicious Mexican narco-terror group, Los Zetas, for help with an attack on a Saudi embassy. The man he approached turned out to be an informant for U.S. drug agents, it says.
The U.S. charges that Arbabsiar had been told by his cousin Abdul Reza Shahlai, a high-ranking member of the Quds Force, to recruit a drug trafficker because drug gangs have a reputation for assassinations.
Iranian lawmakers and analysts have said Iran would not benefit from killing the Saudi ambassador in Washington, and thus has no reason to do so.