LAUSANNE, Switzerland - Iran agreed in principle to accept significant restrictions on its nuclear facilities for at least a decade and submit to international inspections under a framework deal announced Thursday after months of contentious negotiations with the United States and other world powers.
In return, international sanctions that have battered Iran's economy would be lifted in phases if it meets its commitments, meaning it could take a year or less for relief from the penalties to kick in.
The framework agreement, a milestone in negotiations that began 12 years ago, is not a final deal. But it creates parameters for three more months of negotiations over technical details and some matters that remain unresolved. Any one of those issues could doom a comprehensive agreement. Among them is the pace at which sanctions will be suspended.
"The political understanding with details that we have reached is a solid foundation for the good deal we are seeking," said Secretary of State John Kerry, his voice sounding hoarse after an all-night negotiation session.
The agreement includes almost all the restrictions on Iran's nuclear facilities, laboratories, mines, and mills that the United States had sought in recent months, although it initially aimed for even tougher restrictions.
But Iran would get several benefits that may make the deal more palatable to politicians and the public in Tehran. It would not have to close any of its three nuclear facilities, though it would be left with only one that would enrich uranium - at levels low enough to create fuel for power plants but not high enough to create weapons-grade uranium.
The limitations would produce a one-year "breakout" period, meaning it would take Iran a full year to build up enough material for one nuclear warhead, compared with current estimates of two to three months, officials said.
Many sanctions initially would be suspended, rather than lifted permanently as Iran sought, so they could be "snapped back" into place if Iran was discovered to be cheating, the officials said.
Iran's apparent acceptance of so many conditions sought by the United States could give the Obama administration a tool to fend off critics in Congress who want to impose new sanctions to wring more concessions from the Iranians.
The White House fears such steps could scuttle the talks and prompt Tehran to resume its nuclear program at full tilt. Iran maintains its nuclear program is for peaceful, civilian uses.
While the negotiations will continue through June, much of the attention will now shift to the White House and its defense of the negotiations, both in classified briefings to Congress and in public arenas.
President Obama hailed the agreement as a "historic understanding" and asked whether anyone really thinks that the deal is "a worse option than the risk of another war in the Middle East."
The announcement of the agreement was made by weary-looking diplomats from Iran, the European Union, the United States, and five other nations. Most had slept only one or two hours after the previous day's talks, which stretched nearly through the night.
They sounded exuberant even before they arrived at a Lausanne high school a few miles from the hotel where the last rounds of talks had been held. Many diplomats had been cautious after the negotiations failed to meet a March 31 deadline. But once the bargaining ended Thursday, there was a flurry of excited tweets.
"Big day," wrote Kerry, who shouldered most of the direct negotiations with Iran.
Once the agreement was reached, the diplomats walked onto a stage bearing the flags of all the nations involved in the talks - Iran, Germany, France, Britain, Russia, China, and the United States. Then Mohammad Javad Zarif, Iran's foreign minister and chief negotiator in the talks, and the European foreign policy chief, Federica Mogherini, made statements. Mogherini said Iran and the world powers were making a "decisive step" before she listed the main parameters for an eventual deal in which Iran would be permitted to pursue the civilian use of nuclear technology.
Under the agreement, Iran's heavy water reactor in Arak would be rebuilt so it could not produce weapons-grade plutonium. No nuclear fuel would be reprocessed, and spent fuel will be exported or diluted.
Iran's underground plant at Fordow would be converted from a uranium-enrichment site into a nuclear physics and technology center. The site was built secretly deep inside a mountain near Qom and would be difficult to destroy by military attack.
As Mogherini was speaking, the State Department was e-mailing reporters a "fact sheet" outlining more details that it said Iran had agreed to, though it could not immediately be confirmed that Iran was indeed on board with every item.
In one of the most significant ones, the number of Iran's centrifuges would be cut by two-thirds, to about 6,000, according to the statement. It said they would be first-generation machines, not the more advanced ones that Iran has sought. Keeping the old centrifuges is a key element in establishing the one-year breakout period, a red line for Washington.
The restrictions would be monitored through inspections by the International Atomic Energy Agency, and some of them would last 25 years under the accord.
Though six nations negotiated with Iran, much of the heavy lifting was done in meetings between the United States and Iran. The countries have been hostile toward each other for decades, particularly since the 1979 revolution and the seizure of dozens of American diplomats who were held hostage in the U.S. Embassy in Tehran.
But the United States and Iran will remain at odds on many issues.
"We have serious differences with the United States," Zarif said. "We have built mutual distrust in the past. . . . So what I hope is that through courageous implementation of this, some of that trust could be remedied. But that is for us all to wait and see."
Iran agreed to reduce its operating centrifuges by about two-thirds. The centrifuges are to be stored with the International Atomic Energy Agency. The remaining centrifuges will be first-generation, and therefore less efficient, machines.
Iran agreed to not enrich uranium over 3.67 percent of U-235 for at least 15 years. To make a bomb, uranium must be enriched to 90 percent purity.
Uranium enrichment would be limited to one site for 10 years, and no new uranium-enrichment facilities would be built for 15 years.
For the next 15 years, Iran will cut its stockpile of about 22,000 pounds of low-enriched uranium to 661 pounds.
Inspectors will have regular access to all of Iran's nuclear facilities.
Iran would see most of the economic sanctions imposed lifted, though some imposed by the United States would remain.
SOURCE: U.S. State Department