The alleged plot to carry out an assassination on U.S. soil would represent, if proven, a significant escalation of a long-running covert struggle between Iran and the West that has included industrial sabotage, terrorist bombings and the killing of Iranian nuclear scientists.

It also would reflect a radical shift in tactics for a country that usually prefers to leave its dirty work to proxies.

The Obama administration on Tuesday directly accused Iran and its elite Quds Force of backing the alleged attempt to kill Saudi Arabia’s ambassador to the United States, Adel al-Jubeir, using hit men from a Mexican drug gang. The allegation plunged U.S.-Iranian relations into crisis and sent U.S. officials scrambling in search of new punitive measures to impose against a country that has already been hit with multiple rounds of sanctions.

The brazenness of the plot outlined by Justice Department officials struck many current and former U.S. officials as out of character for Iran, which has rarely, if ever, been so bold as to strike targets in America. U.S. officials were similarly surprised last month when an Iranian admiral threatened to send naval ships to patrol off U.S. waters.

“To my mind, it reeks of desperation,” said Matthew Levitt, a former deputy assistant treasury secretary for intelligence and analysis. “It suggests to me that they are feeling cornered.”

To others, the very rashness of the alleged assassination plot raised doubts about whether Iran’s normally cautious ruling clerics supported or even know about it. Robert Baer, a former CIA case officer in the Middle East and author of several books on Iran, said there was “sloppiness about the case that defies belief.”

“Maybe things have really fallen apart in Tehran, or maybe there’s a radical group that wants to stir up the pot,” Baer said. “But the Quds are better than this. If they wanted to come after you, you’d be dead already.”

Other current and former U.S. officials argued that the plot had to have originated at the highest levels of Iran’s government and the elite Revolutionary Guard Corps, given the costs and complexities of conducting such an international operation.

“A reasonable person can conclude that senior members of the [Revolutionary Guard] and Quds Force and the civilian government had to know,” said Mike Rogers (R-Mich.), chairman of the House intelligence committee.

The plot shows that the Iranians “have gotten more and more brazen,” he said. “This was a two-fer for them: kill the Saudi ambassador and embarrass the United States by having it happen on U.S. soil in Washington, D.C.”

Administration officials made clear that they saw the plot as a high-level operation, not the work of rogue agents.

“The United States does not need new reasons to have serious concerns about the Quds Force,” said a senior administration official, citing the militant force’s role in Iraq, Lebanon and other strategic places. “But this plot on U.S. soil is a dangerous escalation, and we consider it a flagrant violation of international law.”

Long before this week, the administration had tense relations with Iran, which snubbed President Obama’s attempts at rapprochement early in his presidency and then proceeded to open new fronts in the 30-year-old cold war between the two countries.

Two months after his inauguration, Obama attempted to appeal directly to the Iranian people in a videotaped message marking the beginning of the Iranian festival of Nowruz. “The United States wants the Islamic Republic of Iran to take its rightful place in the community of nations,” Obama said. “You have that right — but it comes with real responsibilities.”

A month later, Iran sentenced an American-Iranian journalist to prison for spying. Tehran later rejected U.S. proposals for ending the international standoff over its nuclear energy program, which the United States believes is a cover for aspirations for nuclear weapons.

As Iran added inexorably to its stockpile of enriched uranium, the Republican Guard and Quds Force contributed to U.S. casualties in Iraq and Afghanistan, in part by supplying deadly new forms of explosives to insurgents in both countries, Western intelligence officials say.

“In addition to allegedly sponsoring this plot, Iran has supported and provided weapons for attacks on our soldiers in Iraq and Afghanistan,” said Sen. Saxby Chambliss (Ga.), ranking Republican on the Senate intelligence committee. “This has continued far too long with no repercussions.”

The alleged assassination plot would be the first known attempt by Iranians in decades to strike a foreign dignitary on U.S. soil. But Iran has long used proxy forces — most often the Lebanon-based militant group Hezbollah — to carry out targeted killings and terrorist strikes from the Middle East to Europe to South America.

Tehran also is increasingly engaged in tit-for-tat warfare with Western, Israeli and Arab intelligence operatives seeking to undercut Iran’s regional influence and cripple its nuclear program.

The most spectacular blows in the covert war in recent years have fallen on Iran. In the past four years, four Iranian scientists with links to the country’s nuclear program have been slain, by gunshot, bomb or poisoning, by unknown assailants. A fifth scientist barely escaped death in a car-bombing. Iran has accused the United States and Israel of killing the scientists as part of a larger program of sabotage and intimidation aimed at crippling the nuclear program. Neither the U.S. nor Israeli government has commented publicly on the slayings.

Iran’s nuclear program also was damaged by a computer worm that targeted enrichment operations. Some computer experts believe the worm, Stuxnet, was created by the Israelis, perhaps with U.S. help.

In interviews, Iranian officials complain of the damage inflicted by what they say is a black-operations program led by the United States and Israel, and some of them have threatened reprisals. At the same time, Iran faces increasing pressure from long-time rival Saudi Arabia and other Sunni Arab states that are intent on preventing Iran from becoming a nuclear power or expanding its regional influence.

Several Persian Gulf states joined the West in a campaign to isolate Iran’s chief Arab ally, Syrian President Bashar al-Assad. Iran has responded by sending oil, cash and arms to Assad, and by going on the offensive elsewhere. In August, Revolutionary Guard forces also started an intense campaign in the Kurdish border area in an attempt to destroy the Kurdish separatist organization PJAK, which officials in Tehran say is backed by the United States.

But while Iranian officials have acknowledged battling U.S. interests along an array of fronts, they scoffed openly at the new accusations, which a government spokesman on Tuesday dismissed as “big lies that will go nowhere.”

“These sorts of cliche behaviors are based on the old, hostile policies of America and the Zionists,” said Foreign Ministry spokesman Ramin Mehmanparast, “and are just foolish games.”

Staff writers Scott Wilson, Karen DeYoung and Greg Miller and researcher Julie Tate contributed to this report. Erdbrink reported from Tehran.