The Obama administration's key Middle East initiatives — ending Syria's civil war, combating Islamic State and implementing the Iranian nuclear deal — could be undermined by the explosion of tensions between the region's two powerhouses, Saudi Arabia and Iran.
A historically fraught rivalry between Sunni Muslim-dominated Saudi Arabia and Shiite Muslim Iran, previously fought mostly through proxies, now is more direct than ever and threatens to engulf the region in a new spiral of bitter confrontations, analysts said.
In addition to igniting new rounds of vicious fighting in Syria and Yemen, where the two countries back opposing sides, the dispute could play into the hands of Islamic State by further stoking the sectarian conflict it relies on as its raison d'etre, the analysts warned.
The flare-up comes at an awkward time for the Obama administration. The United Nations Security Council could decide within weeks whether Iran is entitled to an easing of international sanctions, and a return to the global economy, under the controversial nuclear accord.
Saudi Arabia took the drastic measure of cutting diplomatic ties with Iran on Sunday after Shiites infuriated by the execution of a prominent Shiite cleric in Saudi Arabia torched the Saudi Embassy in Tehran.
The crisis widened Monday as Saudi allies Bahrain and Sudan also severed relations with Iran. The United Arab Emirates, a major trading partner with Iran, withdrew its ambassador but did not break diplomatic ties.
"This exceptional step has been taken in the light of Iran's continuous interference in the internal affairs of gulf and Arab states, which has reached unprecedented levels," the UAE said in a statement.
Saudi Arabia also barred its citizens from traveling to Iran and suspended air traffic and other commercial relations, although it said Iranians would still be welcome to make the annual religious pilgrimage to Mecca, Islam's holiest city.
Iran must behave "like a normal country" and not "a revolution," Saudi Foreign Minister Adel Jubeir said, according to Reuters.
The Saudi kingdom's decision last week to execute an outspoken Shiite cleric and government critic, Sheik Nimr al-Nimr, along with 46 other dissidents and militants, apparently caught Washington by surprise.
The Obama administration had worked hard to bring Saudi Arabia and Iran into nascent negotiations aimed at finding a political solution to the civil war in Syria. Iran has backed Syrian President Bashar Assad, while Saudi Arabia is supporting some of the armed groups fighting to oust him.
U.N.-backed peace talks are still expected to start this month, but the long-shot prospects for a diplomatic breakthrough now appear considerably dimmer.
"It was essential to bring Saudi Arabia and Iran together, and there was some progress," said Hussein Ibish, senior resident scholar at the Arab Gulf States Institute in Washington. "That is all sort of shot. This is going to complicate just about everything the administration is trying to do" in the Middle East.
Josh Earnest, the White House press secretary, acknowledged Monday that the escalating conflict poses problems for U.S. efforts to find a diplomatic solution to the war in Syria.
"There will always be reasons ... to be reluctant to engage," Earnest said. But the White House is "hopeful" that won't happen, he said, because "it is so clearly in the interest of both countries."
The administration is urging both sides to "de-escalate" their conflict and "not further inflame" tensions, Earnest said.
Secretary of State John F. Kerry spoke by phone Monday to his Iranian counterpart and was attempting similar contact with the Saudis, spokesman John Kirby said.
"Actions like this don't do anything to help stability in the region," Kirby said of the flare-up. He said the U.S. had raised concerns about the Saudi legal process that sanctioned the mass execution.
The European Union's top diplomat, Federica Mogherini, also spoke to Iranian and Saudi leaders, warning that "the security and stability of the whole region … is at stake," her office said.
Saudi Arabia and its gulf Arab allies view Iran with growing distrust and anger, and worry that the U.S.-led nuclear deal brokered in Vienna last summer will allow Iran to end its isolation without giving up its nuclear ambitions.
Under the terms of the deal, if Iran gets rid of its enriched uranium stockpiles and dismantles or disables most of its nuclear infrastructure, it will gain access to more than $60 billion in frozen funds as early as this month, and be allowed to resume exports of oil on the open market.
A more powerful Iran, combined with low prices for the oil that supports the Saudi economy, and the largely unsuccessful war Saudi Arabia is waging against what it claims are Iranian-backed Shiite militias in Yemen, has pushed the kingdom to act more aggressively.
Saudi leaders did not publicly oppose the nuclear deal, but experts say they have lost confidence in the United States' willingness to oppose what they consider Iranian aggression across the region.
"The Saudis have their own agenda, which they are carrying out without regard for what we say or do or need," said Aaron David Miller, a veteran U.S. diplomat in the Middle East who is now with the nonpartisan Wilson Center in Washington.
Iranian First Vice President Eshaq Jahangiri said Saudi Arabia would suffer more from the cutting of diplomatic ties.
"I advise the Saudi leaders to stop these subversive, hasty, illogical, emotional acts that are marked by mismanagement," Jahangiri said during a news conference Monday in Tehran, according to the Iranian media outlet Press TV.
Although the tensions were being ratcheted up at a brisk pace, several experts said they did not expect hostilities to escalate to open warfare.
"I don't believe [Al-Nimr's execution] will be a make or break issue for open war," said Joas Wagemakers, an expert on Sunni-Shiite relations at Utrecht University in the Netherlands.
"Nimr was not openly connected nor overly loyal to Iran, and he did not believe an Islamic state along Iranian lines should be founded in Saudi Arabia."
Still, Al-Nimr will be viewed as a martyr for religious Shiites and would probably become a galvanizing figure for opposition to the Saudi government, Wagemakers said.
Times staff writer Wilkinson reported from Washington and special correspondent Bulos from Dubai, United Arab Emirates. Staff writer Christi Parsons in Washington and special correspondent Ramin Mostaghim in Tehran contributed to this report.