A U.S. grand jury has indicted two Iranian men accused of being involved in an alleged plot to kill the Saudi ambassador to the United States.

The indictment Thursday in New York means the grand jury believes there is enough evidence against Manssor Arbabsiar and Gholam Shakuri to warrant a trial.

Arbabsiar, a naturalized U.S. citizen, is scheduled to enter his plea in court on Monday. Shakuri, who the U.S. says is a member of Iran's elite Quds force, has not yet been arrested.

The U.S. Justice Department says Arbabsiar has confessed to his role in the plot to assassinate the Saudi envoy, possibly by setting off an explosion at a restaurant.

Iran has denied being involved in any plot, and Iranian President Mahmoud Ahmadinejad has accused the United States of fabricating it in order to cause a rift between Tehran and Saudi Arabia.

Supreme Leader Ayatollah Ali Khamenei said Sunday that any “inappropriate” political or security-related action by the U.S. on the issue will bring a “decisive response.”

The U.S. held a face-to-face meeting with Iranian officials about the alleged plot in order to make it “absolutely clear” that Washington considers such behavior “unacceptable” and a violation of U.S. and international law.