JERUSALEM -- For Israel and Persian Gulf states like Saudi Arabia, President Barack Obama's historic telephone call with President Hasan Rouhani of Iran on Friday was the geopolitical equivalent of discovering your best friend flirting with your main rival.
Although few nations have a greater interest in Mr. Obama's promise to stop Iran from developing a nuclear bomb, his overtures to Mr. Rouhani were greeted with alarm here and in other Middle East capitals allied with the United States. They worry about Iran's sincerity, and fear that Mr. Obama's desire for a diplomatic deal will only buy Iran time to continue a march toward building a nuclear weapon.
But beyond that, the prospect of even a non-nuclear Iran -- strengthened economically by the lifting of sanctions, and emboldened politically by renewed relations with Washington -- is seen as a dire threat that could upend the dynamics in this volatile region.
One gulf academic in a Twitter post likened the phone call to "the fall of the Berlin Wall." An Israeli lawmaker said in a radio interview that he hoped Mr. Obama would not be the next Neville Chamberlain, known for appeasement of the Nazis in 1938.
"There is a lot of suspicion and even paranoia about some secret deal between Iran and America," said Jamal Khashoggi, a prominent Saudi journalist who is close to the royal family. "My concern is that the Americans will accept Iran as it is -- so that the Iranians can continue their old policies of expansionism and aggression."
Saudi Arabia and the other Sunni-dominated gulf countries share a concern about a shift in the balance of power toward Iran's Shiite-led government and its allies. For Israel, Iran remains the sponsor of global terrorism and of the Lebanese militia Hezbollah and the Palestinian militant group Hamas, both avowed enemies of Israel's existence.
"They can change the regime, but one thing won't change, and that is the hostility against Israel," warned Uzi Rabi, chairman of a Middle East studies center at Tel Aviv University. "Part of the plan is to drive a wedge between Americans and Europeans and Israel. I hate to say it, but what the Iranians managed to do is to change the whole game."
There was no official reaction Saturday from Riyadh or Jerusalem to the telephone call, which was the first direct conversation between U.S. and Iranian presidents in more than three decades. Prime Minister Benjamin Netanyahu of Israel spent the day rewriting the speech he is scheduled to deliver Tuesday at the United Nations and preparing for a meeting Monday with Mr. Obama. After years in which Mr. Netanyahu exploited Iran's nuclear ambitions to rally the world against Iran and force its isolation, Israel could find itself increasingly isolated in its hard-line stance.
Saudi Arabia and other gulf states view Iran as a regional nemesis whose nuclear program is only one element of a broader effort to project power. The rivalry is made more bitter by the sectarian dimension, and competition over supplying oil to the world. The president's overtures to Iran add to a growing impatience and exasperation among Arabs in the gulf over Washington's retreat from threats to strike Syria, whose civil war is viewed as a proxy for the larger sectarian and strategic battle unfolding across the region.
"The gulf states, and the Iranians, still see this as a balance-of-power struggle," said Salman Shaikh, director of the Brookings Doha Center. "And Obama's warning and Rouhani's charm offensive, as well as what they would see as a hoodwinking of the United States on the nuclear issue, could have far-reaching consequences on the balance-of-power struggle."
Mustafa Alani, a Dubai-based security analyst, said the Saudis think that Mr. Obama is "not a reliable ally, that he's bending to the Syrians and Iranians." Mishaal al-Gergawi, a political analyst based in the United Arab Emirates, said.
Israeli analysts, too, worry over what they see as the Obama administration's weak and wavering policies toward the Middle East. After the Syrian chemical weapons crisis, some said the phone call only upped the ante for a diplomatic victory that could lead Washington to accept what Jerusalem would consider a "bad deal" with Iran, which insists its nuclear program is for civilian purposes only.
"Obama is interested in showing foreign-policy success because he hasn't had too many of them," said Emily Landau, an Iran expert at the Institute for National Security Studies in Tel Aviv. "I'm afraid that for the sake of that he might be willing to compromise on the nuclear issue in a manner that I think is detrimental to U.S. national security interests, leave aside Israel."
The skeptics' main concern now is that four to six months of negotiations would allow Iran to get to the breakout point for developing a bomb.