During Friday prayers in Tehran last summer, one of the ruling system's most firebrand clerics, Ahmad Jannati, had worshippers pumping their fists to denounce Saudi Arabia and other Arab foes across the Gulf.
In response, the Saudi interior minister, Prince Nayef bin Abdul-Aziz, blamed Iran for fomenting street protests and "targeting" the kingdom with unrest.
Mudslinging between the Middle East's two most powerful rivals is a longtime political fact of life. But it has grown much thicker amid the Arab Spring uprisings and now threatens to veer into crisis mode after U.S. allegations arose of an Iranian-aided plot to assassinate Saudi Arabia's ambassador in Washington.
Deeper suspicions and sharper recriminations don't necessarily increase the risk of a new Mideast conflict, which would invariably draw in U.S. forces in the Gulf. The higher tensions, however, are likely to raise temperatures in current flashpoints _ such as Syria and Bahrain _ where Saudi and Iranian views collide.
"The Saudi-Iran rivalry is the pivot point for so much in the region," said David Schenker, a Mideast affairs analyst at the Washington Institute for Near East Policy. "What is happening now could set the tone for years or decades."
Saudi policy makers could double down on efforts to further isolate Iran by boosting assistance to Syrian opposition forces seeking to topple Tehran's main ally, Bashar Assad. In the past week, Iran felt further squeezed as neighboring Turkey agreed to host a NATO missile defense radar and the European Union added 29 more Iranian officials to its sanctions list.
In London, a former Saudi ambassador to the U.S., Prince Turki al-Faisal, said "somebody in Iran will have to pay a price." The prince's aides said his remarks were personal views and did not reflect official policy.
The official Saudi news agency said the kingdom strongly condemned what it called the "evil, heinous" plot, describing it as a violation of international law.
Iran has dismissed the assassination plot claims as "American propaganda." But that didn't slow the potshots at Saudi Arabia, which many Iranian officials view as rubber-stamp envoys for U.S. interests in the region.
"This is in line with policy of divide and rule," said Iranian lawmaker Farhad Bashiri, addressing an open session of parliament Wednesday. "Saudi Arabia should be careful not to fall in the U.S. trap."
Theodore Karasik, a security expert at the Dubai-based Institute for Near East and Gulf Military Analysis, said the Justice Department allegations offer Saudi Arabia more ammunition to increase its regional drive against Iran.
"I think they'll make a lot of noise," said Karasik. "And a lot of the other (Gulf) states will make a lot noise, saying, `This is more Iranian meddling, and now it's gone out of the region all the way to America, and we all need to be more careful.'"
The grievance list is long on both sides.
Saudi Arabia and its Gulf Arab allies _ all Sunni Muslim regimes with deep Western ties _ are deeply concerned over any gains by Shiite power Iran.
In one of the most repeated snippets from leaked U.S. diplomatic cables, Saudi's King Abdullah in 2008 urged a U.S.-led attack against Iran to "cut off the head of the snake" and halt Tehran's nuclear program.
The Gulf Arabs had to swallow the rise of Iranian-allied Shiite factions in Iraq after the U.S.-led invasion. But they drew the line with Bahrain, a tiny Gulf kingdom that is home to the U.S. Navy's 5th Fleet. A Saudi-led military force entered Bahrain in March to help the Sunni monarchy crush pro-reform protests by the Shiite majority.
Iran denounced the Saudi intervention as an "occupation" using Riyadh's huge arsenal of U.S. weapons, which could be further boosted by a proposed $60 billion purchase that includes 84 new F-15 fighter jets and 190 helicopters.
Iran also portrays Saudi Arabia as dangerous ground for key figures. An Iranian scientist, Shahram Amiri, claims he was kidnapped by the CIA in Saudi Arabia in 2009 and interrogated in the U.S. about Iran's nuclear program, which Washington and its allies fear could lead to atomic weapons. Iran says it only seeks reactors for energy and research.
U.S. officials said Amiri willingly came to the U.S. and the CIA paid him a total of $5 million to provide intelligence, but Amiri did not take the money with him back to Iran.
Saudi Arabia has its own accounts of Iranian-linked intrigue.
In 1989, Saudi Embassy employees were killed or seriously injured during a series of attacks at the kingdom's missions in Turkey, Belgium and Lebanon. Pro-Iranian groups active in Beirut at the time claimed responsibility for the assassinations.
In Bangkok, meanwhile, three Saudi diplomats and another embassy employee were murdered in 1989 and 1990. Thai authorities later accused pro-Iranian group known as Jund al-Haqq _ Arabic for "soldiers of justice" _ of carrying out those attacks. The group had previously claimed responsibility for one of the slayings in 1989.
Thailand's interior minister at the time said the killings were to avenge the deaths of Iranian pilgrims in a 1987 clash with Saudi police in the Islamic holy city of Mecca that killed about 400 people. But authorities in Thailand reopened the case in 2007.
The U.S. allegations, made public Tuesday, say two suspected Iranian agents tried to recruit a purported Mexican hit man _ actually a paid informant for the U.S. Drug Enforcement Administration _ to kill Saudi Ambassador Adel Al-Jubeir, a U.S.-educated former adviser sent to Washington by King Abdullah after the Sept. 11, 2001, attacks to help repair the kingdom's image.
Fifteen of the 19 9/11 hijackers were from Saudi Arabia. Al-Jubeir was appointed ambassador in 2007.
A dual Iranian-American national, Manssor Arbabsiar, was arrested last month and charged along with fugitive Gholam Shakuri, who authorities claim is a member of an elite wing of Iran's Revolutionary Guard known as the Quds force.
But some experts who study Iranian covert tactics say the alleged conspiracy does not bear the hallmarks of the Quds force, which favors use of established proxy factions such as Lebanon's Hezbollah and would not take risks alleged by the U.S. such as wiring money to a regular bank account.
"Iran has been known to carry out pre-operational surveillance in the United States, but it has not yet used this intelligence to carry out a high-profile attack," said a report by the international intelligence think tank Stratfor. "It seems unusual that the Iranians would approach a Mexican cartel to carry out the assassination when the Iranians probably have the capability themselves.
Associated Press writers Adam Schreck in Dubai, United Arab Emirates, and Ali Akbar Dareini in Tehran, Iran, contributed to this report.