Federal agents have disrupted a major plot involving a "significant terrorist act in the United States," federal officials told ABC News Tuesday. The attack had ties to Iran, they said.

The Saudi and Israeli embassies in Washington, D.C. were the intended targets of the bomb plot, and Saudi Arabian ambassador to the U.S. Adel Al-Jubeir was supposed to be assassinated, officials added.

The same countries' embassies, abroad in Buenos Aires, Argentina, were also possible targets.

DEA and FBI agents have been working to crack what they called "Operation Red Coalition" since May, when an Iranian-American man in Texas, Manssor Arbabsiar, contacted an undercover DEA informant and told him of a plan to assassinate the Saudi ambassador.

Arbabsiar offered $1.5 million in exchange for help with the assassination, and had already wired $100,000 as a down payment before he was apprehended.

Arbabsiar said that he had been "directed by high-ranking members of the Iranian government," sources told ABC News, and federal agents suspected that the man may be linked to members of a special forces unit in the Revolutionary Guard.

Arbabsiar also allegedly told the DEA informant that he could provide "tons of opium" to the Mexican drug cartels whose help he hoped to enlist in the assassination and bombings.

Another Iranian man charged in connection with the terror plot, Gholam Shakuri, is a member of the Qods Force, the special forces unit of Iran's Revolutionary Guard.

Shakuri remains at large.

Arbabsiar is being held by federal authorities in New York. According to a Department of Justice statement, Arbabsiar and Shakuri are charged with conspiracy to murder a foreign official; conspiracy to engage in foreign travel and use of interstate and foreign commerce facilities in the commission of murder-for-hire; conspiracy to use a weapon of mass destruction (explosives); and conspiracy to commit an act of international terrorism transcending national boundaries.

Arbabsiar is also charged with another count of foreign travel and use of interstate and foreign commerce facilities in the commission of murder-for-hire.

U.S. Attorney General Eric Holder held a press conference Tuesday afternoon, and released a statement about the plot.

"The criminal complaint unsealed today exposes a deadly plot directed by factions of the Iranian government to assassinate a foreign Ambassador on U.S. soil with explosives," Holder said. "Through the diligent and coordinated efforts of our law enforcement and intelligence agencies, we were able to disrupt this plot before anyone was harmed. We will continue to investigate this matter vigorously and bring those who have violated any laws to justice."

A spokesman for Iranian president Mahmoud Ahmadinejad denied the Iranian government's involvement in a statement to CNN.

"It's not the first time that America has come up with a story like this. America is facing domestic problems and this is an attempt by them to distract the public by trying to convince that there is an outside threat," the spokesman said. "From our perspective, this is a fabrication. America has become an expert at making false allegations against other countries."

The criminal complaint is filed in New York. The prosecution of the case is led by Assistant U.S. Attorneys Glen Kopp and Edward Kim, of the Terrorism and International Narcotics Unit of the U.S. Attorney’s Office for the Southern District of New York, with assistance from the Counterterrorism Section of the Justice Department’s National Security Division.

Watch this video from CNN's Newsroom, broadcast Oct. 11, 2011.

This post was edited after publication to add additional information.