Cairo: Almost as soon as he had announced a nuclear deal with Iran, President Barack Obama called King Salman of Saudi Arabia to reassure him of America’s “enduring friendship".

Returning the courtesy, Salman, who is Iran’s chief regional rival, responded that he hoped the deal would “reinforce the stability and security of the region and the world", the Saudi Press Agency reported on Friday.

But the picture on the ground was not so harmonious.

As Tehran and its clients around the Arab world celebrated the accord as a triumph of Iranian resolve, Saudi Arabia and its allies declared the agreement had only reinforced their determination to push back against Iranian influence, with or without Washington. On the front lines of battles with Iranian proxies in Syria and Lebanon, some cried betrayal.

“The Saudi king decided his country could no longer bear the provocative Iranian expansive policy in the Middle East, or the American silence over it," wrote Jamal Khashoggi, a veteran Saudi journalist and former government adviser, in a commentary this week on what he called “the Salman doctrine".

“Saudi Arabia no longer cares if this silence is the passing weakness of a president whose term ends in two years," he wrote, “or if it is a conspiracy or a major deal that President Barack Obama is negotiating with the Iranians."

On Friday, Khashoggi’s column was translated into English on the front page of the website of the Saudi-owned network Al Arabiya.

The opposition between Washington and Iran has been an organizing principle of regional affairs for 35 years, wrapping the oil-rich Persian Gulf monarchies in a blanket of US military protection and determining the balance of political and military power among competing Iranian-and Western-backed factions across Syria, Iraq, Lebanon and the Palestinian territories.

Now, the prospect of US-Iranian cooperation on a nuclear deal threatens to upend all that. Amid an escalating power struggle between the two regional heavyweights, Iran and Saudi Arabia, the unquestioned allegiance of the world’s only superpower no longer seems so axiomatic. Some question how Iran might respond, and factions across the Arab world scrambled on Friday to make sense of who wins and who loses.

Moving quickly to reassure US allies, the White House released a statement saying Obama had told Salman that the understanding with Iran “will not in any way lessen US concern about Iran’s destabilizing activities in the region". The president also invited the king and the leaders of the other Persian Gulf monarchies to meet at Camp David this spring “to continue consultations".

“The agreement does not reduce fears," a sub-headline on Al Arabiya’s website declared.

“Of course, things did not change," said a Saudi diplomat, speaking on condition of anonymity, following protocol. “The agreement addresses one aspect but not the whole issue of Iranian expansionism."

For the ninth straight night, Saudi jets were conducting a bombing campaign against the Iran-backed Houthi movement in Yemen, an operation widely billed in Saudi Arabia as a message to Tehran that the Arab states can put up a fight, even without Washington.

The US announced after the fact that it would provide intelligence and logistical support to the Saudi-led campaign, though the Obama administration views the Houthis as an independent movement that takes money from Iran rather than an instrument of Iranian influence.

In Syria, where Iran has sustained President Bashar Assad in a proxy war against mostly Sunni Muslim rebels backed by the Persian Gulf monarchies, many in the opposition denounced the deal as a betrayal of Obama’s stated support for their cause.

Aboud Dandachi, an anti-government activist now living in Turkey, bitterly suggested that the Sunni Islamist militants who recently captured the Syrian city of Idlib follow Iran’s example: “Set up a theocracy in Idlib, fund terror groups worldwide, and then Obama will give you your heart’s desire," he wrote in a Twitter message about the deal.

Monzer Akbik, a representative of the Syrian opposition coalition in exile, said the lifting of sanctions on Tehran would mean more cash to support Assad.

“If the news we are hearing is true, that $150 billion in Iranian deposits will be released from banks soon, this will help Assad a lot," he said. “We fully object to any agreements signed at the expense of Syria." Bloomberg

Hwaida Saad and Anne Barnard in Beirut, Omar al-Jawoshy in Baghdad and Merna Thomas in Cairo contributed to this story.