Updated

A man charged in a plot to assassinate the Saudi ambassador to the United States has pleaded not guilty in a New York City courtroom.

Manssor Arbabsiar, who did not speak during the proceeding, entered a not guilty plea on each of the five counts charged against him, which include conspiracy to murder a foreign official and conspiracy to use a weapon of mass destruction.

According to the Department of Justice, Arbabsiar -- a naturalized US citizen who holds both Iranian and US passports -- allegedly confessed to planning the assassination when he was arrested by federal agents at JFK International Airport in New York City on Sept. 29.

In court Monday, Arbabsiar, 56, was dressed in blue prison clothing and his gray beard had noticeably grown since his last court appearance earlier this month, the New York Post reports. At least one Saudi diplomat attended the hearing, but he declined to comment to the newspaper.

The arraignment lasted just five minutes. Arbabsiar's next court appearance was scheduled for Dec. 31.

The other suspect in the case, Gholam Shakuri, remains at large, presumably in Iran. Shakuri is allegedly an Iran-based member of Iran's Quds Force special operations unit.

Arbabsiar and Shakuri were each indicted last Thursday in connection with the $1.5 million assassination plot allegedly directed by elements of the Iranian government.

Arbabsiar allegedly contracted with men he believed were Mexican drug cartel associates to set off explosives at a public restaurant in Washington, D.C., where Adel al-Jubeir, the Saudi Arabian ambassador to the US, was to be dining. The men turned out to be paid informants of the U.S. government.

A paid confidential informant told federal investigators that Arbabsiar said that it was "no big deal" when he was informed there could be as many as 150 casualties, including U.S. Senators known to frequent the establishment, as a result of the restaurant bombing.

Earlier this month, authorities in the U.S. and UK froze the assets of Iranian-based Quds Force members Qasem Soleimani, Hamed Abdollahi and Abdul Reza Shahlai for their alleged roles in the plot.

The Iranian Foreign Ministry has called the assassination-for-hire accusations "baseless."

Newscore and the Associated Press contributed to this report.