U.S. officials on Tuesday accused elements of the Iranian government of plotting to assassinate the Saudi ambassador to Washington, allegations that aggravated the tense relationship between the United States and the Islamic Republic.

The Justice Department unsealed charges against two Iranians — one of them a U.S. citizen — accusing them of orchestrating an elaborate murder-for-hire plot that targeted Adel al-Jubeir, the Saudi envoy to Washington and a key adviser to King Abdullah. The Iranians planned to employ Mexican drug traffickers to kill Jubeir with a bomb as he ate at a restaurant, U.S. officials said.

Attorney General Eric H. Holder Jr. said that “the United States is committed to holding Iran accountable for its actions,” but other officials indicated that it was not yet clear who in the Iranian government was behind the alleged plot.

“There’s a question of how high up did it go,” said an administration official who spoke on the condition of anonymity to discuss internal White House thinking. “The Iranian government has a responsibility to explain that.”

Federal authorities said they foiled the plan because the Iranian American, Mansour Arbabsiar, happened to hire a paid informant for the Drug Enforcement Administration to carry it out. Arbabsiar, 56, was arrested Sept. 29 in New York and later implicated Iranian officials in Tehran in directing the plot, U.S. officials said.

In addition to Jubeir, officials said, the plot envisioned later striking other targets in the United States and abroad, including a Saudi embassy, though those plans appeared preliminary at best. Arbabsiar has acknowledged that he was recruited and funded by men he understood to be senior officers in the Quds Force, an elite division of Iran’s Revolutionary Guard Corps responsible for foreign operations, court documents say.

An Iran-based member of that force, Gholam Shakuri, is also charged in federal court in New York with conspiracy to murder a foreign official and to commit an act of international terrorism, along with other counts.

The Iranian government denied the accusations, calling them a new round of “American propaganda” and saying they were fabricated to divert attention from U.S. economic troubles and the Occupy Wall Street protests.

“The U.S. government and the CIA have very good experience in making up film scripts,” Ali Akbar Javanfekr, a spokesman for Iranian President Mahmoud Ahmadinejad, said in Tehran. “It appears that this new scenario is for diverting the U.S. public opinion from internal crises.”

Ali Larijani, Iran’s former top nuclear negotiator and current head of parliament, described the U.S. allegations Wednesday as “silly and mischievous.” He told the semiofficial Mehr News Agency: “They made noises that they arrested those who wanted to bomb the Saudi Embassy. Foolish words that, by using expanded media coverage, were clearly meant to cover up their internal problems.” He added: “We have normal relations with the Saudis. There is no reason that Iran wants to do these childish things they accuse us of.”

Quick U.S. reaction

The Treasury Department also announced financial sanctions against Shakuri, three other Quds Force officials and Arbabsiar, and Secretary of State Hillary Rodham Clinton said the administration would work with allies to devise more actions to isolate the Islamic Republic.

The State Department also issued a worldwide travel alert for U.S. citizens over the suspected Iranian plot.

“The U.S. government assesses that this Iranian-backed plan to assassinate the Saudi ambassador may indicate a more aggressive focus by the Iranian government on terrorist activity against diplomats from certain countries, to include possible attacks in the United States,” the department said in a statement on its Web site. The travel alert expires Jan. 11, 2012.

Arbabsiar appeared Tuesday in U.S. District Court in Manhattan and was ordered held without bail in a proceeding that did not require him to enter a plea. His court-appointed attorney, Sabrina Shroff, said outside the court that he would plead not guilty, Bloomberg News reported. Shakuri remains at large in Iran.

Shiite-dominated Iran and Sunni Muslim Saudi Arabia are longtime rivals for regional dominance, a contest that moved into high gear with the U.S. elimination of Iraq’s Saddam Hussein as a powerful buffer between them and began to play out in proxy battles in Lebanon, Bahrain and elsewhere.

But some specialists on Iran expressed skepticism that the Islamic Republic would resort to killing a prominent Saudi official — a virtual act of war against that country — in the U.S. capital.

“Why would Iran want to assassinate the Saudi ambassador in Washington?” said Alireza Nader, an Iran expert at the Rand Corp. “I’m not discounting the evidence necessarily, and Iran has a long history of supporting terrorism. But plots against the Saudi ambassador in Washington, D.C., would be outside that norm.”

Other experts said the seemingly unusual method of carrying out the assassination — recruiting what the plotters thought was a Mexican drug trafficker — made sense. “Let’s face it: The level of scrutiny in Mexico is less,” said Fred Burton, a former State Department security specialist who monitors threats in Mexico for the Stratfor group.

Mexican drug cartels are now multifaceted, transnational criminal organizations that have developed increasingly sophisticated car bombs. U.S. federal agents have said the Mexican mafia’s learning curve — from crude pipe bombs to radio-triggered plastic explosives — has been rapid.

U.S. officials declined to comment on Iran’s motive for the alleged plot, saying that the information is classified and that they are continuing to investigate. They also would not specify the other possible targets, declining to confirm other media reports that they included the Saudi and Israeli embassies in Washington.

‘A deadly plot’

Officials described the details of the plan as chilling, with FBI Director Robert S. Mueller III saying that “though it reads like the pages of a Hollywood script, the impact would have been very real, and many lives would have been lost.’’

The plot dates to early spring, when a cousin of Arbabsiar’s, a senior member of the Quds Force, approached him while he was in Iran about a plan to kidnap Jubeir, according to court documents. The criminal complaint does not identify the cousin, but a Treasury Department release issued Tuesday said a cousin of Arbabsiar named Abdul Reza Shahlai, a Quds Force official, “coordinated the plot” and approved financial payments.

Arbabsiar allegedly told the cousin that he did business on both sides of the U.S.-Mexican border and knew a number of drug traffickers. The cousin told Arbabsiar that he should hire a trafficker to carry out the plot “because people in that business are willing to undertake criminal activity in exchange for money,” according to the complaint.

It remains unclear what led Arbabsiar to the person identified only as CS-1. The confidential DEA source, referred to by Arbabsiar as “the Mexican” in meetings tape-recorded by the source, was described in court papers only as a paid informant who was once charged in the United States with a drug offense.

The charges were dropped because the informant has provided valuable information in a number of cases, and in this instance, he quickly notified federal agents that Arbabsiar had contacted him, according to court documents and federal officials.

The two began a series of meetings in Mexico in May that quickly turned to discussing the killing of Jubeir, the documents say. Jubeir, the son of a Saudi diplomat, is one of the most powerful foreign policymakers outside the royal family.

The informant told Arbabsiar that he would need four men to carry out the assassination. His alleged price: $1.5 million.

Shakuri, identified by Treasury as a deputy to Shahlai, gave Arbabsiar thousands of dollars to fund the plot, court documents say. As a down payment, Arbabsiar allegedly arranged for nearly $100,000 to be wired to an account that was secretly overseen by the FBI.

For the site of the bombing, the informant suggested a Washington restaurant where Jubeir “goes out and eat[s] like two times a week,” according to the recordings. When the informant noted that bystanders could be killed in the attack, including U.S. senators, Arbabsiar dismissed these concerns as “no big deal,” court documents say.

“They want that guy [the ambassador] done [killed],” Arbabsiar reportedly said. Federal authorities said the informant was never referring to an actual restaurant.

According to the Justice Department, Arbabsiar was arrested by federal agents while on a layover at New York’s John F. Kennedy International Airport after being denied entry in Mexico. Arbabsiar waived his Miranda rights against self-incrimination and has provided “extremely valuable intelligence,” according to a letter prosecutors sent last week to a federal judge in New York updating her on the status of his detention.

A team of law enforcement agents has been working “virtually around the clock since the defendant’s arrest,’’ it said.

Staff writers Jason Ukman, Scott Wilson and Greg Miller, correspondents Thomas Erdbrink in Tehran and William Booth in Mexico City, and staff researcher Julie Tate contributed to this report.