Israel and some Republicans in Congress called the nuclear deal hammered out between Iran and world powers very dangerous, despite assurances from the United States that the pact would make American allies in the region safer.
"What was achieved last night in Geneva is not a historic agreement. It was a historic mistake," Israel's Prime Minister Benjamin Netanyahu told a cabinet meeting Sunday morning. "Today the world become a much more dangerous place, because the most dangerous regime in the world took a significant step towards getting the most dangerous weapon in the world."
The agreement between Iran and the United States, France, Germany, Britain, China and Russia aims to halt the progress of the Iranian nuclear program and rolls back key parts of it.
Struck with great speed given a history of failed negotiations, it also comes less than three months after Iran's new President Hassan Rouhani promised, in an interview with NBC News, to dramatically alter Iran’s relationship with the world.
Earlier, Israel's Deputy Defense Minister Danny Danon called the agreement "extremely dangerous for the free world."
"It goes without saying that all options remain on the table and that Israel has the capability – and the responsibility – to defend itself using any means necessary," Danon said in a statement.
The West and Israel fear that Iran has been seeking to develop a nuclear weapons capability. Tehran denies this, saying its nuclear program is a peaceful energy project.
The White House has tried to reassure Israel that its fear that a deal would leave it vulnerable was unfounded. Late Saturday, President Barack Obama admitted huge challenges remain and said Iran's promises will be put to the test over the next six months.
"As we go forward, the resolve of the United States will remain firm, as will our commitment to our friends and allies - particularly Israel and our Gulf partners, who have good reason to be skeptical about Iran's intentions," he said.
Then, on Sunday, Obama called Netanyahu to discuss the deal and "underscored that the U.S. will remain firm in our commitment to Israel, which has good reason to be skeptical about Iran's intentions," according to a statement from the White House.
And, emphasizing the United States' commitment to Israel as well as his personal relationship with Netanyahu, Kerry said on Sunday that the two allies continue to share the same strategy and the United States will not tolerate a nuclear Iran threatening Israel.
"There is no difference whatsoever between the United States and Israel about what the end goal is,” Kerry said.
Earlier, Kerry said the agreement could not have been reached without the Iranians' decision to come to the negotiating table. He said the next phase of talks, while even more difficult will also be more important
"If this first step leads to what is our ultimate goal – which is a comprehensive agreement – that will make the world safer," he said.
England's Prime Minister David Cameron called the deal "an important first step."
"We now have an international agreement with Iran that moves it further away from getting a nuclear weapon. This is an important first step, which must now be fully implemented," said Cameron."Today's deal with Iran demonstrates how persistent diplomacy and tough sanctions can together help us to advance our national interest."
Russia’s Vladimir Putin called the agreement a victory for both sides and called on all negotiating parties to pursue a long-term solution ensuring Iran’s right to a peaceful nuclear energy program.
"It is only the first step in a long and uneasy path ... (But) the principle of step-by-step (movement) and reciprocity have found reflection in the approved document which has won ... global recognition," Putin said in an email to Reuters.
Spain, Norway and Sweden expressed hope for a more sweeping accord.
"The agreement represents an important step toward the normalization of relations between the international community and Iran, and toward a general agreement that promotes stability and security in the region," Spain said, according to the Associated Press.
Herman Van Rompuy, president of the European Council, called the deal a crucial step toward "providing assurances that guarantee the peaceful nature of the Iranian nuclear program,” according to the AP.
The agreement with Iran will likely also affect U.S. relations with Saudi Arabia, a Sunni nation, which is threatened by signs of improved U.S. relations with Shiite Iran.
Gulf reaction to the announced Iran deal has been divided. Behind the scenes, senior Gulf diplomats have had a host of mixed feelings, sources told NBC News.
The UAE officially has welcomed the deal but at least one senior Gulf diplomat was much more critical and expressed skepticism over the deal.
Others see the U.S. holding secret meetings with Iranian officials in Oman over the past year as “insulting” because the U.S. has pressured Gulf countries like the UAE to step up sanctions on Iran, much to the detriment of their own economic self-interests. For years, the UAE was a major economic hub for Iranian commerce but since sanctions were increased, business has declined.
However, small Gulf states Bahrain and the United Arab Emirates hailed the deal.
“We welcome this agreement it if will end of the (sic) fear of any weapons of mass destruction in the region,” Bahrain foreign minister Khalid bin Ahmed Al Khalifa told reporters in Manama, according to the AP.
Iran’s neighbor to the east, Pakistan, a declared nuclear power, said the accord “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,” and Turkish president Abdullah Gul also lauded the deal.
The deal stipulates that Iran will commit to halt uranium enrichment above 5 percent and also to neutralize its stockpile of near-20 percent uranium. The Islamic Republic has also committed to halt progress on its enrichment capacity. Iran will also halt work at its plutonium reactor and provide access to nuclear inspectors.
In exchange, the United States and its allies have agreed to offer Iran "modest relief" from economic sanctions and access to a portion of the revenue that the country has been denied through these sanctions. No new sanctions will be imposed.
The Obama administration also faces skeptics in Congress. Reaction poured in late Saturday and early Sunday and appeared to be divided along party lines.
Rep. Ed Royce, R-Calif., chairman of the House Foreign Affairs Committee, warned that the deal does not meet the standards necessary to protect the United States and its allies.
"Instead of rolling back Iran’s program, Tehran would be able to keep the key elements of its nuclear weapons-making capability. Yet we are the ones doing the dismantling – relieving Iran of the sanctions pressure built up over years," Royce said in a statement.
House Majority Leader Eric Cantor, R-Va., echoed those concerns, saying in a statement he found it "troubling" that the agreement "still permits the Iranians to continue enriching."
"Iran's long history of noncompliance with the U.N. Security Council is well known, as is its use of secret facilities to pursue its nuclear program," Cantor added.
Sen. Marco Rubio, R-Fla., warned that the deal sets a bad precedent.
"This agreement shows other rogue states that wish to go nuclear that you can obfuscate, cheat, and lie for a decade, and eventually the United States will tire and drop key demands," Rubio said in a statement.
NBC News' Ann Curry, Luke Russert, Ayman Mohyeldin and F. Brinley Bruton contributed to this report.