Six world powers reached an interim agreement with Iran on its disputed nuclear program after four days of talks in Geneva.
In the six-month interim deal, Iran agreed to limit nuclear activities in return for relief from some $6.1 billion in sanctions that have hurt its economy.
Late Saturday night, President Obama called the agreement "an important first step," but said sanctions can be reapplied if the Iranians violate it.
The agreement opens "a new path to a world that is more secure," Obama said in a speech at the White House, adding "Iran cannot use negotiations as cover to advance its (nuclear) program."
According to a White House fact sheet, the interim deal "halts the progress of Iran's nuclear program and rolls it back in key respects." In return, world powers will provide "limited, temporary, targeted and reversible relief" on sanctions to Iran.
Under the agreement, Iran has agreed to stop enriching uranium above 5% and dismantle "the technical connections" required to enrich it above 5%. It will also neutralize its stockpiles of 20% enriched uranium, which experts say represents 90% of the effort required to produce weapons-grade uranium.
U.S. negotiators and their counterparts from the United Kingdom, France, Germany, Russia and China had been meeting with the Iranians since Wednesday in an effort to strike the deal.
The interim agreement was applauded as a substantial step by some nuclear and national security analysts.
"This first-phase agreement ensures that we will continue to negotiate a complete end to Iran's nuclear program and should reassure U.S. allies in the region that Iran cannot make a dash for the bomb," said Joe Cirincione, president of the Ploughshares Fund, a group focused on nuclear non-proliferation.
Dave Solimini, a spokesman for Democratic-leaning Truman Project, a Washington-based national security group, said the interim agreement proves that years of sanctions against Iran have worked.
"These are basic requirements after 30 years of distrust, ensuring that negotiations will not be used to buy time to build a nuclear weapon," Solimni said. "Iran's sincerity in these negotiations must always be subject to proof, which is why ongoing inspections and surveillance of their facilities is even more important now. The secondary sanctions which were switched off can just as easily be switched back on if Iran fails to keep their word."
The last round of talks broke down over French concerns about the status of Iran's heavy water power plant under construction in Arak, and over Iran's demand that any agreement recognize the production of nuclear fuel as Iran's sovereign right. The USA does not recognize such a right.
World powers have imposed crippling economic sanctions on Iran to persuade it to suspend production of nuclear fuel in accordance with United Nations Security Council resolutions, and to prove its nuclear program is peaceful in compliance with the Non-Proliferation Treaty, which Iran has signed.
Iran, which seeks to have the sanctions lifted, says its nuclear program has peaceful aims. But the U.N.'s nuclear watchdog, the International Atomic Energy Agency, has reported over the years multiple discoveries of secret Iranian nuclear sites, work on nuclear detonators and implosion devices with assistance from foreign scientists, documents describing safety arrangements for a nuclear test, and plans for a spherical payload for Iranian missile.
The United States, Israel and European countries have said they suspect Iran of developing a nuclear weapons program. President Obama has said the USA will prevent Iran from obtaining a nuclear weapon and Israel has threatened to use military force to prevent what it considers an existential threat.