Manssor Arbabsiar, 56, standing in his navy blue prison uniform, entered the plea in U.S. District Court in Manhattan to a five-count indictment returned last week.
Those charges were the same as those contained in an Oct. 11 criminal complaint. According to the complaint, Arbabsiar has admitted his role in a $1.5 million plot to kill the ambassador at a restaurant by setting off explosives.
President Barack Obama's administration has accused agents of the Iranian government of being involved in the plot. The press attache at Iran's mission to the United Nations has called the accusation "baseless."
No trial date has been set. U.S. District judge John F. Keenan set Dec. 21 for Arbabsiar's next court appearance, saying the weeks until then would give prosecutors sufficient time to turn over evidence in the case and for the defense to prepare a likely challenge to whether the government gave Arbabsiar adequate warning of his legal rights before questioning him.
Arbabsiar's lawyer, Sabrina Shroff, declined to comment after Monday's proceeding, which lasted only several minutes.
Arbabsiar was arrested Sept. 29 at John F. Kennedy International Airport on charges that carry a potential life sentence. He was held without bail.
U.S. authorities say they secretly recorded conversations between Arbabsiar and an informant with the Drug Enforcement Administration after Arbabsiar approached the informant in Mexico and asked his knowledge of explosives for a plot to blow up the Saudi embassy in Washington. They said Arbabsiar later offered $1.5 million for the death of the ambassador.
Authorities said Arbabsiar eventually made a $100,000 down payment wired from an overseas account for a plot that would kill the ambassador at his favorite restaurant.
