Israel's prime minister has harshly condemned the international community's nuclear deal with Iran while Saudi Arabia has remained conspicuously quiet, reflecting the jitters felt throughout the Middle East over Iran's acceptance on the global stage.
Elsewhere, many welcomed the agreement as an important first step toward curbing Iran's suspect nuclear programme.
Israel and Western-allied Gulf countries led by Saudi Arabia have formed an unlikely alliance in their opposition to Sunday's deal, joined together by shared concerns about a nuclear-armed Iran and the Tehran's growing regional influence.
While most Gulf countries remained silent in the first hours after the deal was reached in Geneva, Israeli Prime Minister Benjamin Netanyahu wasted little time in criticising it, calling it a "historic mistake" and saying he was not bound by the agreement.
Speaking to his Cabinet, Netanyahu said the world had become a "more dangerous place" as a result of the deal. He reiterated a long-standing threat to use military action against Iran if needed, declaring that Israel "has the right and the duty to defend itself by itself."
Sunday's agreement is just the first stage of what is hoped to bring about a final deal ensuring that Iran does not develop a nuclear weapon.
Under the deal, Iran will curb many of its nuclear activities for six months in exchange for limited and gradual relief from painful economic sanctions. The six-month period will give diplomats time to negotiate a more sweeping agreement.
The package includes freezing Iran's ability to enrich uranium at a maximum 5 percent level, which is well below the threshold for weapons-grade material and is aimed at easing Western concerns that Tehran could one day seek nuclear arms. International monitors will oversee Iran's compliance.
For Iran, keeping the enrichment programme active was a critical goal. Iran's leaders view the country's ability to make nuclear fuel as a source of national pride and an essential part of nuclear self-sufficiency.
But Israel views any enrichment as unacceptable, saying making low-level enriched uranium weapons grade is relatively simple. It demands all enrichment be halted, and that Iran's abilities to produce uranium be rolled back.
Netanyahu had also called for economic sanctions to be increased. Israel fears that Iran will use the diplomatic process as cover to trick the international community, much the way North Korea did in its march toward a nuclear bomb.
"Today the world became a much more dangerous place," Netanyahu said.
Israeli officials acknowledged they would have to turn their focus toward affecting the outcome of the final negotiations. Israel is not part of the Geneva talks but remains in close touch with the US and other participants.
Israel feels especially threatened by Iran, given Tehran's repeated references to destroying Israel, its support for hostile militant groups on Israel's borders and its development of long-range missiles.
Israeli President Shimon Peres, a Nobel Peace laureate, expressed cautious optimism that Sunday's deal could change the region.
"I would like to say to the Iranian people: You are not our enemies and we are not yours. There is a possibility to solve this issue diplomatically. It is in your hands. Reject terrorism. Stop the nuclear programme. Stop the development of long-range missiles," he said.
Another Nobel peace laureate, Mohammed ElBaradei, Egypt's pro-democracy leader and former director of the United Nations nuclear watchdog agency, welcomed the deal.
In a tweet on his official account, he wrote: "After decade of failed policies, world better off w/ Iran deal. Equity, trust building, respect & dialogue R key to any conflict resolution."
The muted response in the Gulf came after the rulers of Qatar and Kuwait met Saudi King Abdullah over the weekend to discuss regional issues, foremost Iran.
Saudi Arabia and Iran's regional enmity increasingly has played out as a proxy war in Syria with both countries providing lethal support for the warring sides. Saudi Arabia also accuses Iran of backing Shiite unrest across the region.
Bucking the trend, the tiny Gulf countries of Bahrain and the United Arab Emirates praised the agreement. Hamas called the deal a sign of Iran's rising power and prominence.
"We welcome this agreement if it will the end of the fear of any weapons of mass destruction in the region," Bahrain Foreign Minister Khalid bin Ahmed Al Khalifa told reporters in Manama.
Across Europe, countries welcomed the deal. Britain and France, which both participated in the negotiations, said they were committed to seeing the deal succeed, while Spain, Norway and Sweden expressed hope for a broader solution.
"The agreement represents an important step toward the normalisation of relations between the international community and Iran, and toward a general agreement that promotes stability and security in the region," Spain said.
Herman Van Rompuy, president of the European Council, called it an important step toward "providing assurances that guarantee the peaceful nature of the Iranian nuclear program."
Russian President Vladimir Putin, whose government also participated in the talks, said the agreement vindicated Russia's calls for a diplomatic solution.
"The result of Geneva is a win for all, showing once again that by working collectively and with mutual respect it is possible to find answers to current international challenges and threats," Putin said in a statement released by the Kremlin.
Iran's eastern neighbour Pakistan, a declared nuclear power, said the deal "should augur well for peace and security in our region and the world at large." Turkey, which borders Iran to the west, called it a "new start."
Pakistan's rival India, another nuclear power, also welcomed the deal.
Iran's allies, meanwhile, lined up behind the deal.
Iraqi government spokesman Ali al-Moussawi called it a "step forward in order to solve other regional problems."
The Syrian government, which relies on Iran's support in its battle against rebels fighting to topple President Bashar Assad, also said it was proof that negotiations were the best way to resolve a conflict. The Western-backed Syrian opposition, the Syrian National Council, said they feared the deal would give Iran "legitimacy" to keep up its support of Assad's forces in Syria.