A US grand jury on Thursday formally indicted two men suspected of being part of an Iranian plot to kill the Saudi ambassador to Washington.

US authorities announced last week they had foiled a plot by two men linked to Iran's security agencies to assassinate Saudi Ambassador Adel al-Jubeir.

One suspect was arrested in the United States last month and the other is believed to be in Iran.

Manssor Arbabsiar, 56, was arrested on September 29 when he arrived at John F. Kennedy International Airport from Mexico.

US officials say the other man, Gholam Shakuri, is a member of Iran's shadowy Quds Force, the covert operations arm of Iran's powerful Islamic Revolutionary Guard Corps.

Both were initially charged in a criminal complaint.

Arbabsiar will have a chance to enter a plea to the charges on Monday, when he is scheduled to be arraigned. His defense attorney, Sabrina Shroff, has previously stated her client would plead not guilty.

Details that have emerged in the past week - such as Arbabsiar's apparently bumbling nature and his approach to a supposed Mexican drug cartel figure who happened to be a US federal informant - have raised eyebrows among Iran specialists as to the seriousness of the plot.

Yet the consensus view in US President Barack Obama's administration is that Iran's supreme leader, Ayatollah Ali Khamanei, probably knew of the alleged plot to kill Saudi's ambassador in Washington, while President Mahmoud Ahmadinejad did not.

The two men are charged with one count of conspiracy to murder a foreign official, two counts of use of interstate and foreign commerce facilities in the commission of murder for hire and one count each of conspiracy to use a weapon of mass destruction and conspiracy to commit an act of terrorism.

US District Judge John Keenan will oversee the case in Manhattan federal court.-Reuters