President Barack Obama hailed an historic interim deal with Iran Saturday as "an important first step" towards a comprehensive pact to end the showdown over Teheran's nuclear programme.
The president however quickly ran into criticism from opponents on Capitol Hill, reflecting the deep mistrust of Tehran after more than 30 years of Cold War-style hostilities and Obama's own ragged domestic political fortunes.
"While today's announcement is just a first step, it achieves a great deal," Obama said in a late night statement from the White House after the agreement was clinched in talks between P5+1 world powers and Iran in Geneva.
"For the first time in nearly a decade, we have halted the progress of the Iranian nuclear programme, and key parts of the programme will be rolled back."
The president also warned that if Iran did not live up to its side of the bargain, it would both lose the nearly $7 billion in sanctions relief allowed under the interim deal and face increased sanctions.
Washington and its allies had taken "an important first step toward a comprehensive solution that addresses our concerns with the Islamic Republic of Iran's nuclear programme," Obama said.
Senior US officials said the deal did not "recognize" Iran's right to enrich uranium, and committed Tehran to take a number of steps to halt progress on its nuclear programme.
These include, according to the White House, a commitment to halt uranium enrichment above five percent and to neutralize a stockpile of 20 percent enriched uranium.
The White House said that Iran agreed to not to install additional centrifuges or to use any of its new generation centrifuge, and to leave inoperable roughly half of installed centrifuges at the Natanz plant and three-quarters of installed centrifuges at Fordow.
Tehran also agreed to not commission its Arak reactor, designed to eventually produce plutonium that could be used in a nuclear bomb, and to halt construction on the plant.
In return, world powers, including the United States, Russia, China, Britain, France and Germany plus the European Union, agreed to a set of "modest" measures to ease sanctions that have choked the Iranian economy.
Obama stressed that the "toughest" sanctions would remain pending a final deal.
"We will refrain from imposing new sanctions, and we will allow the Iranian government access to a portion of the revenue that they have been denied through sanctions," Obama said.
Obama's political allies will likely frame the deal as a victory for a president who has seen his global standing erode over an National Security Agency spying scandal, and who has struggled to reach a coherent policy towards Syria's use of chemical weapons.
Obama's Iran policy however faces intense opposition from Republicans and some Democratic hawks on Capitol Hill, an early hint of the tough sales job he and Secretary of State John Kerry face.
"The President sees wisdom in placing trust, however limited, in a regime that has repeatedly violated international norms and put America's security at risk," said Howard 'Buck' McKeon, the Republican chairman of the House Armed Services Committee.
Republican Senator Marco Rubio added that "by allowing the Iranian regime to retain a sizable nuclear infrastructure, this agreement makes a nuclear Iran more likely."
"There is now an even more urgent need for Congress to increase sanctions until Iran completely abandons its enrichment and reprocessing capabilities," he said.
Ed Royce, chairman of the House Foreign Affairs committee criticized the sanctions relief under the deal as "more lifeline than 'modest.'"
In his speech, Obama warned lawmakers planning to impose new sanctions on Iran that such a move could "derail" the progress over the next six months designed at getting a comprehensive deal.
Much of the criticism from Capitol Hill mirror concerns expressed by Israel -- something Obama acknowledged when he said that he recognized that the interim deal would make some US allies nervous.
US resolve "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.
White House officials said they expected Obama to call Israel Prime Minister Benjamin Netanyahu on Sunday to brief him on the deal.
Despite the criticism, the deal is a vindication for Obama's strategy of engaging Iran, including his telephone call with new Iranian President Hassan Rouhani in September. If a final deal can be reached it will likely be the centerpiece of his foreign policy legacy.
"I have a profound responsibility to try to resolve our differences peacefully, rather than rush towards conflict," Obama said.
"Today, we have a real opportunity to achieve a comprehensive, peaceful settlement, and I believe we must test it."