BAGHDAD • Saudi Arabia's decision to cut diplomatic ties with Iran marks a swift escalation in a strategic and sectarian rivalry that underpins conflicts across the Middle East.
It came at a time when the United States and others had hoped that even limited cooperation between the two powers could help end the crushing civil wars in Syria and Yemen, while easing tensions in Iraq, Bahrain, Lebanon and elsewhere.
Instead, analysts feared it would increase sectarian divisions and investment in proxy wars.
"This is a very disturbing escalation," said Mr Michael Stephens, an analyst at the Royal United Services Institute, a London-based research centre. "It has enormous consequences for... the region, and the tensions between the two sides are going to mean that instability across the region will continue."
US officials have said the Saudi-Iranian split does not bode well for international peacemaking efforts that require the two powers to make compromises.
The US called for dialogue, with State Department spokesman John Kirby saying: "We believe that diplomatic engagement and direct conversations remain essential in working through differences."
US Secretary of State John Kerry spoke to Iranian Foreign Minister Mohammed Javad Zarif on Sunday. Officials would not describe the contents of the call, but it was clearly an effort to urge the Iranians not to escalate the situation by retaliating.
Still, the prospects for accommodation appeared to have reached their lowest point in years.
Saudi Arabia and Iran follow separate strands of Islam, and have long been rivals for influence across the Middle East and beyond. That rivalry has accelerated in recent years as the Iraq War and the Arab Spring uprisings upturned the regional order and gave both countries new ways to extend their reach.
That put them on opposite sides of various conflicts, often divided by sect. In Bahrain, Saudi Arabia sent tanks to support the Sunni monarchy against protesters led by the island-nation's Shi'ite majority. In Syria, Iran has bankrolled President Bashar Assad's government, while Saudi Arabia has backed Sunni rebels seeking his ouster. In Yemen, the Saudis have led an air campaign against Shi'ite Houthi rebels.
Further straining tensions are Saudi concerns that the Iranian nuclear accord could increase Teheran's ability to spread its influence. And Iran remains angry over Saudi Arabia's handling of a stampede during the haj last September that left more than 2,400 pilgrims dead, including more than 450 Iranians, according to an Associated Press count.
But setting off the war of words that finally broke relations was Saudi Arabia's execution of Shi'ite cleric Nimr al-Nimr, who had acted as a spiritual leader for protesters from the kingdom's Shi'ite minority. The government accused him of inciting violence and executed him with 46 others, mostly Al-Qaeda members.
Analysts said the split could further destabilise the region.
"These countries don't trust one another, and they see every event as an opportunity to raise tensions," said Mr Abbas Kadhim, a senior foreign policy fellow at the School for Advanced International Studies at Johns Hopkins University.
Since Saudi Arabia and Iran both appear reluctant to attack each other directly, he is worried that they would increase their investment in indirect confrontations elsewhere.
"Both countries will try their best to try to fortify their proxies and their activities, which is going to create more trouble," he said.
That risks derailing a new round of international peace talks aimed at ending the civil war in Syria, a process Mr Kerry has worked hard to get going. The talks, meant to begin this month, were to be the first to bring together the Syrian government, the opposition and a range of countries that include Iran and Saudi Arabia.
NEW YORK TIMES