WASHINGTON — The Obama administration on Wednesday sought to reconcile what it said was solid evidence of an Iranian plot to murder Saudi Arabia’s ambassador to the United States with a wave of puzzlement and skepticism from some foreign leaders and outside experts.

Senior American officials themselves were struggling to explain why the Quds Force, an elite international operations unit within Iran’s Islamic Revolutionary Guards Corps, would orchestrate such a risky attack in so amateurish a manner.

The White House spokesman, Jay Carney, would not go further than to say the plot “clearly involved senior levels of the Quds Force.” But other American officials, armed with evidence such as bank transfers and intercepted telephone calls and with knowledge of how the covert unit operated in the past, said they believed that Iran’s senior leaders were likely complicit in the plot.

“It would be our assessment that this kind of operation would have been discussed at the highest levels of the regime,” said a senior American official, who was not authorized to speak publicly about the government’s analysis.

American officials offered no specific evidence linking the plot to Iran’s most senior leaders. But they said it was inconceivable in Iran’s hierarchy that the leader of the shadowy Quds Force, Maj. Gen. Qassim Suleimani, was not directly involved, and that the country’s supreme leader, Ayatollah Ali Khamenei, was not aware of such a plan.

Iran’s leaders marshaled a furious formal rejection Wednesday of the American accusations, calling the case a cynical fabrication meant to vilify Iran and distract Americans from their severe economic problems. A senior member of Iran’s Parliament, Alaeddin Boroujerdi, said he had “no doubt this is a new American-Zionist plot to divert the public opinion from the crisis Obama is grappling with.”

United States officials said they were exploring several theories why the Quds Force, which supplies and trains insurgents around the world, would plot an attack in Washington against a close adviser to the Saudi king, relying on an Iranian-American used-car salesman from Texas who, they said, thought he was hiring assassins from a Mexican drug gang.

The officials said the plot might indicate a shift to a more combative Iranian foreign policy toward Saudi Arabia and the United States. The United States has brought international pressure on Iran’s nuclear program, and Iran and Saudi Arabia have long waged proxy battles for influence in the Muslim world.

“The Iranians watch the Saudis roll tanks in Bahrain, and they see a key ally in Syria going down, so they step up the Quds Force,” one senior administration official said. He referred to Saudi military assistance to the Sunni monarchy of Bahrain, whose majority population shares the Shia Islam of Iran.

Iran has many trusted networks in the Middle East and has often used the Lebanese militants of Hezbollah as a proxy. But it has far fewer agents in the United States, which might have forced it to look to a far riskier proxy for the plot, officials said.

American investigators have speculated that the Iranian-American accused in the scheme, Mansour J. Arbabsiar, who lived in Texas on the Mexico border, may have convinced a cousin, a senior Quds official, that he could recruit a member of one of Mexico’s notorious drug cartels to carry out the killing.

One provocative theory that American officials are considering is that the assassination was intended as retaliation for the killing of several Iranian nuclear scientists during the past two years. Those deaths are widely believed to have been the work of Israel, with tacit American approval, to slow Iran’s progress toward a nuclear weapon.

In a protest letter denying the American charges late Tuesday, Iran’s ambassador to the United Nations, Mohammad Khazaee, referred pointedly to the assassination campaign. “Iran has been a victim of terrorism,” he wrote, “a clear recent example of which is the assassination of a number of Iranian nuclear scientists in the past two years carried out by the Zionist regime and supported by the United States.”

An American official said of Iranian officials that “certainly their publicly expressed anger at the death of some of their scientists could have been part of their calculation.” But the official said the United States government had no specific evidence to support that theory.

Juan Zarate, a deputy national security adviser for combating terrorism in the Bush administration, said that if the upper echelons of the Quds Force had approved the operation, it also must have been approved by Ayatollah Khamenei, President Mahmoud Ahmadinejad or both.

If that was the case, he said, it crossed what he called a “red line,” bringing to American soil a proxy sectarian war between Shiites and Sunnis that Iran has been fighting with Saudi Arabia for influence in the region in places like Syria and Bahrain.

But Mr. Zarate and senior American officials said the assassination plan did not have the hallmarks of a Quds operation. “It was very extreme and very odd, but it was also very sloppy,” Mr. Zarate said. “If you look at what they have done historically, they can put operatives on their targets and execute. They usually don’t outsource, but keep things inside a trusted network.”

One problem for President Obama and his administration is that since American intelligence claims about Iraq’s illicit weapons proved false in 2003, assertions by the United States about its adversaries have routinely faced skepticism from other countries.

“Of course, that is in people’s heads. Everyone is extremely skeptical about U.S. intelligence revelations,” said Volker Perthes, an Iran expert who is the director of the German Institute for International and Security Affairs in Berlin.

There may indeed have been a plot, Mr. Perthes said. “I don’t regard it as impossible but rather improbable,” he said.

If the Iranian leadership authorized such a plot, he said, that would mark “a major escalation against the United States, of the kind that hasn’t happened since the Iranian Revolution.” It would be “almost an act of war,” and Washington “must react in a different way than it has so far.”

Alain Frachon, a Le Monde columnist and a former Washington correspondent, said that “we can expect anything from a regime as split and divided as the Iranian regime is,” adding that “one group among them is probably capable of launching such an operation to embarrass the others.”

While the United States’ history with Iraq might color the European reaction, Mr. Frachon said, he is “not sure you’ll find the same amount of skepticism in Paris as there was with W.M.D. In the case of Iraq, it was easier to assess, but Iran is much more opaque, and people are willing to expect anything from a divided regime.”

Steven Erlanger contributed reporting from Paris, Jo Becker from New York, and Anthony Shadid from Beirut, Lebanon.