GENEVA — The euphoria over the signing of a historic nuclear agreement with Iran gave way to sober realism Sunday as the parties clashed over a key element of the deal and congressional skeptics threatened to thwart it.
The Obama administration moved quickly to sell the agreement to nervous US allies, particularly Israel, and to persuade lawmakers not to push ahead with new economic sanctions that could prompt Iran to abandon the six-month freeze on its nuclear program.
In televised interviews, Secretary of State John F. Kerry defended the deal, asserting that the United States and its allies believe that it ensures Iran will either abide by the agreement or face the reinstatement of measures that have crippled Iran's economy.
"We have no illusions. We don't do this on the basis of somebody's statements to you. We do it on the basis of actions that can be verified," Kerry told CNN.
Kerry also acknowledged that keeping the deal on track could ultimately prove to be more arduous than the task of securing the landmark agreement in the first place.
"The next phase, let me be clear, will be even more difficult, and we need to be honest about it," Kerry told reporters after the nuclear pact's first phase was approved by diplomats from Iran and six major powers. "But it will also be even more consequential."
The deal, sealed in a predawn signing ceremony Sunday in Geneva, freezes or reverses progress at all of Iran's key nuclear facilities and bars it from adding new centrifuges. It freezes enrichment of uranium at 5 percent, and freezes or eliminates stockpiles of uranium that Western officials fear could be turned into fuel for a nuclear weapon.
Iran also agreed to unprecedented daily monitoring of its nuclear program by international inspectors.
It will be allowed to keep its nuclear infrastructure, but it pledged not to finish construction of a controversial heavy-water reactor in Arak that could also provide it with a source of plutonium for a nuclear bomb if the government decided to pursue one. Iran has long insisted that its nuclear program is intended for peaceful, energy-producing purposes.
The Israeli prime minister, Benjamin Netanyahu, harshly condemned the deal on Sunday. He called it a ''historic mistake'' and said his government was not bound by its terms.
Speaking to his Cabinet, Netanyahu said the world had become a ''more dangerous place'' as a result of the deal. He reiterated a longstanding threat to use military action against Iran if needed.
Saudi Arabia remained quiet, reflecting concerns of many countries in the region over Iran's acceptance on the global stage. Israel, Saudia Arabia, and other Western-allied countries in the region have formed an unlikely alliance in their opposition to easing sanctions against Iran.
In Tehran, officials welcomed the deal as the beginning of a new era for the Islamic Republic, with President Hassan Rouhani asserting that language in the agreement affirmed Iran's right to enrich, which he and other top officials had demanded as an element of any agreement.
"Let anyone make his own reading, but this right is clearly stated in the text of the agreement that Iran can continue its enrichment, and I announce to our people that our enrichment activities will continue as before," Rouhani said on live television Sunday morning.
Kerry and other US officials — who had sought to find language that would satisfy Iran without implying legal recognition of a right to enrich — disputed that interpretation.
"There is no inherent right to enrich," Kerry said on ABC's "This Week," in an apparent reference to the Non-Proliferation Treaty, which is silent on whether a country's right to pursue nuclear energy for peaceful purposes also allows for the enrichment of uranium. "Everywhere in this particular agreement it states that they could only do that by mutual agreement, and nothing is agreed on until everything is agreed on."
The Geneva pact received a significant boost on Sunday as a key Senate proponent of Iran sanctions indicated he would defer imposition of additional measures to see if Iran keeps its promises.
But Senator Robert Menendez, Democrat of New Jersey, chairman of the Senate Foreign Relations Committee, said he believed that the agreement did not go far enough in rolling back Iran's nuclear program, and he said the Senate would not "abstain from preparations to impose new sanctions on Iran, should the talks fail."
Menendez said new sanctions legislation being considered by the Senate would include a six-month window to allow diplomats to try to negotiate permanent limits to Iran's nuclear program.
But other prominent lawmakers, including Senator Chuck Schumer, Democrat of New York, said they would consider pushing for new sanctions in defiance of the Obama administration.
The agreement reached in Geneva "makes it more likely that Democrats and Republicans will join together and pass additional sanctions when we return in December," Schumer said.
Other lawmakers and conservative think tanks slammed the pact for granting Iran economic relief that many said was out of proportion to Tehran's nuclear concessions.
Although the agreement would keep the harshest economic sanctions in place, some analysts argued that any significant relief would lead to a collapse in the system of economic restraints that have cut Iran's oil exports in half and decimated the country's currency.
"Iran has broken the back of the Western sanctions regime," said Mark Dubowitz, a sanctions expert and director of the Foundation for Defense of Democracies, a Washington-based think tank. "It is an illusion to believe that sanctions will not be eroded significantly by this deal."
The Obama administration, which has been holding direct talks with Iran on the issue for most of the year, is already preoccupied with the next steps.