WASHINGTON -- Iran has agreed to terms of an initial bargain set by world powers in the hopes of pausing a standoff with the United States and Europe over its nuclear program.

"Diplomacy opened up a new path toward a world that is more secure," US President Barack Obama said in a late-night address from the White House, characterizing the deal as the "most significant and tangible progress" since the beginning of his presidency.

"For the first time in nearly a decade, we have halted the progress of the Iranian nuclear program," Obama said. "And key parts of the program will be rolled back."

Iran's decision on Sunday morning to comply with the deal — characterized in recent days by the Israeli government as "very, very bad" — was hailed in Geneva, Switzerland, as the "first step" toward a peaceful solution to the decade-old crisis.

"Because of this agreement, Iran cannot use negotiations as cover to advance its program," Obama said, adding that the deal "cuts off [Iran's] paths to a bomb."

Germany and the five permanent members of the United Nations Security Council — the United States, United Kingdom, France, Russia and China — announced the interim agreement after the chief diplomat of each nation descended on Geneva, where talks were in their third round since October, in order to secure the deal.

United Nations Secretary-General Ban Ki-moon heralded the deal as "what could turn out to be the beginnings of a historic agreement."

Iranian President Hassan Rouhani on Sunday congratulated Foreign Minister Mohammed Javad Zarif and the six world powers involved in reaching an interim agreement after four long days of negotiations in Geneva.

"Iranian people's vote for #moderation & constructive engagement (plus) tireless efforts by negotiating teams are to open new horizons," he posted on Twitter, after re-Tweeting Zarif's announcement of the deal.

US Secretary of State John Kerry said in the early hours of Sunday morning that the historic deal "enlarges the breakout time" Iran would need to develop fissile material for a nuclear device.

"The purpose of this is very simple: to require Iran to prove the peaceful nature of its program," Kerry said, "and to ensure that it cannot build a nuclear weapon."

Kerry stated that the deal would make America's regional allies including Israel, safer, and the US president echoed that sentiment in his remarks from the State Dining Room of the White House.

"As we go forward, the resolve of the United States will remain firm, as will our commitments to our friends and allies –- particularly Israel and our Gulf partners, who have good reason to be skeptical about Iran’s intentions," Obama said.

Addressing Prime Minister Binyamin Netanyahu's concerns with the basic effectiveness of an interim deal, a senior US administration official said on Saturday night in a phone call with journalists that "we weren't going to get to an end state from a standing start."

"The prime minister has raised concerns in the past about the growing stockpile of 20 percent uranium this would neutralize that stockpile," the official said. "We and the Israelis were concerned about the Arak reactor coming online; we believe this halts Arak in its tracks for the first time."

The 'first step' imposes a limit on Iran’s enrichment capabilities and dilutes existing stockpiles of uranium, effectively halting parts of the program that are most worrisome to the international community. The deal specifically addresses the enrichment of uranium to weapons-grade quality, spun in centrifuges to such a degree that the material has no practical civilian purpose.

Iranian Foreign Minister Zarif said at a Sunday morning press conference in Geneva that Iran will never stop enriching uranium. He told reporters that enrichment will continue and "will be a part of any agreement now and in the future," although halting it is a significant portion of the agreement. Although many restrictions will be implemented over the next six months, it will not completely cease.

He continuously insisted that is it Iran's "inalienable right" and urged other countries other countries to "recognize and respect those who decide by their own free will" to develop nuclear technology. He added that the international community should "refrain from imposing restrictions, when we are exercising our right."

When asked about Israel's potential reaction to the deal, Zarif avoided naming the country, but said that there is no reason to react negatively. "The deal is geared toward resolving a problem that has cast its shadow cast over entire world, and this region. I do not see any justification to be concerned about the resolution of a problem."

"We are trying to move forward with the international community," he added. "They must accept fact that threat of war is illegal. War is unnecessary, imprudent and illegal. If we can prevent that, it is an accomplishment. The force option is no longer on the table."

He later told Iranian Press TV that the west had finally "recognized" Iran's nuclear rights, and it would only be a matter of time until all sanctions were lifted, once they address some outstanding concerns.

Iran will grant "unprecedented transparency" to not just its nuclear plants, but also to its ​raw materials at uranium mines and mills, which have previously had no formal oversight, a senior administration official said on Saturday night.

International monitors will be granted unfettered access to Iran's largest nuclear facilities, including Fordow, a uranium enrichment plant burrowed deep inside a mountain in the holy city of Qom and kept clandestine until 2009.

In his statement on Sunday morning, Kerry recalled that the revelation of Fordow's existence reinforced international suspicion over Iran's nuclear intentions, and led to the existing sanctions regime.

Additionally in the deal, three-quarters of all centrifuges already installed at Fordow, and half of those at Natanz — another major facility — will be rendered inoperable within the coming months.

Iran will not be allowed to install new centrifuges, but will be required under the agreement to freeze all enrichment beyond 20 percent — a key step towards weaponization. The Iranian government will also be required to "dilute" all uranium already enriched above 5 percent into a form "not suitable for further enrichment," the White House said on Saturday night.

But Iran may continue enriching uranium to 3.5 percent, and is required to dismantle any existing enrichment infrastructure.

The heavy-water plutonium reactor in Arak — which provides the Islamic Republic with another path to a nuclear warhead — will not be commissioned, fueled or further expanded.

In exchange for these concessions from Tehran, world powers will provide the Islamic Republic with sanctions relief valued up to $7-10 billion, including $4.2 billion in direct foreign exchange. The sanctions adjustments are "limited and reversible," says the US government.

The official said that sanctions relief on Iran would amount to, "at most, $6-7 billion dollars," in return for "very important concessions."

That compares to over $100 billion lost in oil revenue alone over the last two years, the official noted, and an expected $30 billion more over the next six months, throughout the duration of the interim deal.

"The relief automatically expires at the end of six months," the official noted, adding that the relief offered will be tightly controlled and "doled out in increments."

The White House released a list of sanctions that would remain in place immediately after the announcement of the deal, which includes those against the Central Bank of Iran, Iran's oil sector and restricted access to the US financial system.

The agreement will be in effect for six months, during which time the P5+1 powers will attempt to forge a conclusive, final-status agreement that will end the nuclear impasse.

"Nothing will be agreed to unless everything is agreed to," Obama said.

The US government has said its goal is to "put time on the clock" in order to stave off military conflict between itself with its allies and the Iranian government.

US officials have characterized an alternative strategy popular on Capitol Hill — not to strike an interim deal with Iran, but rather to sanction its government even further — as equivalent to ordering a march to war.

"This deal appears to provide the world's leading state sponsor of terrorism with billions of dollars in exchange for cosmetic concessions that neither fully freeze nor significantly roll back its nuclear infrastructure," said Senator Mark Kirk (R-Ill), who favors new sanctions against Iran and lobbied against the deal. "Furthermore, the deal ignores Iran's continued sponsorship of terrorism, its testing of long-range ballistic missiles and its abuse of human rights." Kerry canceled a trip to Israel earlier in the week in order to clear his schedule for the weekend, expecting his presence would be required in Geneva.

The secretary will fly on from the Swiss city to London to meet with UK Foreign Secretary William Hague on Iran, and other matters, including Syria and the Middle East peace process.

The international community has refused to accept Iran's production of fissile material since 2003, when George W. Bush called the Iranian government part of an "axis of evil" for its attempts to build weapons of mass destruction.

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