American-Iranian national Mansour J. Arbabsiar has been slapped with a maximum 25-year jail time by a New York federal judge on Thursday for his 2011 plot to murder Adel al-Jubeir, Saudi Arabia's ambassador to the United States.

Mr Arbabsiar planned to kill Mr al-Jubeir at Cafe Milano, an upscale Georgetown restaurant, through a bombing assassination. In October 2012, the almost 60-year-old used-car salesman from Texas pleaded guilty to a charge of murder-for-hire and two counts of conspiracy with Iranian military elements to hire assassins from the Mexican drug mafia to kill the Saudi envoy.

Mr Arbabsiar "fully realized his act. He must learn the lesson. That cannot be tolerated," New York federal Judge John Keenan said when he handed down the sentence. Initially, his defense lawyers argued Mr Arbabsiar suffered from bipolar disorder.

Mr Keenan said he noticed the American-Iranian national was actually fully competent as he even dismissed his lawyers' request for a sentence of just 10 years.

"Whatever I did wrong, I take responsibility for it," Mr Arbabsiar addressed the judge before his sentenced was handed down. "I can't change what I did. I have a good heart. I never hurt anyone."

"My mind sometimes is not in a good place."

During the trial, prosecutors stressed Mr Arbabsiar was a member of the Quds Force, a sub-unit of Iran's Revolutionary Guard Corps, which is closely aligned with Iran's supreme leader, Ayatollah Ali Khamenei. The Quds Force is the one tasked with "exporting" Iran's Islamic revolution, as well as "extraterritorial operations" of the Revolutionary Guard.

According to a 1998 document by the Federation of American Scientists, the primary mission of the Quds Force is to organise, train, equip, and finance foreign Islamic revolutionary movements. It also maintains and builds contacts with underground Islamic militant organisations throughout the Islamic world.

Mr Arbabsiar was arrested on Sept. 29, 2011, at Kennedy International Airport.

Mr Arbabsiar gave $1.5 million to a member of the Los Zetas Mexican drug cartel to plant a bomb at a Washington restaurant that would kill Mr Adel al-Jubeir while he dined.

However, the plot turned sour when a man in Mexico whom Mr Arbabsiar was an associate of a drug cartel to hire a team of assassins, turned out to be a confidential tipster for the Drug Enforcement Administration.

When told the bombing assassination could implicate or kill 100 to 150 other people, "No problem," or "No big deal," was Mr Arbabsiar's reply to their conversation, that was secretly taped by the drug agency's informer.

"It is an extraordinary crime that requires an equally serious sentence," prosecutor Glen Kopp told the judge.