Iran Nuclear Deal: Tentative Agreement Reached

— -- After eight straight days of high-level diplomatic negotiations between six world powers and Iran, negotiators in Lausanne, Switzerland, announced an interim agreement has been reached on the principles for dismantling Iran's nuclear program in exchange for lifting heavy economic sanctions.

"Today we have taken a decisive step," said Federica Mogherini, the High Representative of the European Union, responsible for coordinating the talks. "We have reached solutions on key parameters of a joint comprehensive plan of action."

At the White House, President Obama called it “an historic understanding with Iran, which, if fully implemented” will prevent the country from obtaining a nuclear weapon.

“This has been a long time coming,” Obama said.

Mogherini outlined some specifics about the interim deal in a the news conference, standing alongside Iran's foreign minister.

Among the agreements:

Iran's deep-buried nuclear facility at Fordo will be converted from a a nuclear site into a physics and technology center. No fissile material will remain there. Iran has agreed not to enrich uranium over 3.67 percent for at least 15 years and to reduce its current stockpile of about 10,000 Kg of low-enriched uranium to 300 Kg (a 98% reduction) for 15 years. It's stockpile of enriched uranium will have to be either diluted or sold in the international marketplace.

The heavy water reactor at Arak will be redesigned and will not produce weapons-grade plutonium that could be used to make a bomb.

Iran’s breakout timeline -- the time that it would take for Iran to acquire enough fissile material for one weapon -- would be increased from 2 to 3 months to at least one year for a duration of 10 years. Iran has also agreed to not build any new facilities for the purpose of enriching uranium for 15 years.

In return, many of the United Nations and European nuclear-related economic sanctions against Iran will be lifted immediately following verification that Iran has taken the steps agreed upon steps to dismantle its nuclear hardware. Other sanctions, including those approved by Congress, will be lifted in a phased approach.

"If Iran cheats, the world will know it. If we see something suspicious, we will inspect it," President Obama said. "With this deal, Iran will face more inspections than any other country in the world."

Moments before the news conference, Iranian leaders, including President Hassan Rouhani and Foreign Minister Mohammad Javad Zarif, released short statements via Twitter today that "solutions on key parameters" have been "reached" and that negotiators are ready to begin drafting an agreement.

U.S. Secretary of State John Kerry tweeted moments after the Iranians that negotiators "now have parameters to resolve major issues" on Iran's nuclear program. "Back to work soon on a final deal," Kerry wrote.

"We have said from the beginning -- and I think you've heard me say it again and again -- that we will not accept just any deal, that we will only accept a good deal," Kerry said later, speaking at a solo press conference in Lausanne. "And today, I can tell you that the political understanding, with details, that we have reached is a solid foundation for the good deal that we are seeking."

Although the outline presented in Lausanne does not represent the final stage of the deal, it is the most significant step yet towards a nuclear accord with Iran and marks a major milestone in diplomatic relations with Iran, which have been in a deep freeze for over 35 years.

Negotiators now have to begin drafting a more detailed, formal agreement with a goal to have it signed by all parties by the end of June.

Negotiators on all sides have been working to get to this point since November 2013. That's when Iran and the so-called P5+1 (U.S., U.K., France, Russia, China plus Germany) agreed on a Joint Plan of Action, an interim agreement that paved the way for talks by temporarily halting Iran's nuclear enrichment program and subjecting it to daily inspections in exchange for the loosening of some economic sanctions.

But the Obama administration's most difficult negotiations may occur back in Washington, where just two weeks ago, 367 members of Congress, many of them Democrats, signed a letter expressing "grave and urgent" concern over the Iran nuclear negotiations.

"So when you hear the inevitable critics of the deal sound off, ask them a simple question," President Obama said, addressing his detractors in Congress. "Do you really think that this verifiable deal, if fully implemented, backed by the world's major powers, is a worse option than the risk of another war in the Middle East?"

Though Congress does not have to sign off on an executive-level agreement, it could seek to derail the accord by voting to enact tougher economic sanctions on Iran. President Obama has threatened to veto that legislation, but it's possible Congress could get the two-thirds majority needed to override a presidential veto.

Sen. Tom Cotton, R-Arkansas, a fierce critic of the Iran negotiations and author of the now-infamous congressional letter to Iran's supreme leader, wasted no time saying he rejects this most recent agreement and will work with his colleagues to "stop this deal from going forward."

"This deal is going to threaten America's national security interests and it's going to lead to a nuclear arms race in the most volatile region in the world," Cotton said in an interview with ABC News today.

Get real-time updates as this story unfolds. To start, just "star" this story in ABC News' phone app. Download ABC News for iPhone here or ABC News for Android here.