Iran, the United States and other world powers struck a historic deal Tuesday to curb Iranian nuclear programs in an agreement aimed at easing the threat of a nuclear-armed Iran in the volatile Middle East. In exchange, Iran will get billions of dollars in relief from crushing international sanctions.

The accord, reached after long, fractious negotiations, marks a dramatic break from decades of animosity between the United States and Iran, countries that have called each other the "leading state sponsor of terrorism" and "the Great Satan."

"This deal offers an opportunity to move in a new direction," President Barack Obama said in early morning remarks from the White House that were carried live on Iranian state television. "We should seize it."

Cautious optimism expressed by most politicians 2:40

In Tehran, Iranian President Hassan Rouhani said "a new chapter" has begun in his nation's relations with the world. He maintained that Iran had never sought to build a bomb, an assertion the U.S. and its partners have long disputed.

Beyond the hopeful proclamations from the U.S., Iran and other parties to the talks, there is deep skepticism of the deal among U.S. lawmakers and Iranian hardliners. Obama's most pressing task will be holding off efforts by Congress to levy new sanctions on Iran or block his ability to suspend existing ones.

House Speaker John Boehner, a Republican, predicted the deal would embolden Iran and fuel a nuclear arms race around the world. It will be difficult for congressional Republicans to stop Obama, however, because of his power to veto legislation.

Israel, which sees Iran as a threat to its existence, strongly opposes leaving the Islamic republic with its nuclear infrastructure in place. Israeli Prime Minister Benjamin Netanyahu, who has furiously lobbied against a deal, called the agreement a "stunning historic mistake."

In a phone call Tuesday, Obama sought to reassure Netanyahu that the agreement doesn't diminish U.S. concern about Iran's threats toward Israel and its support for terrorism, the White House said. Netanyahu's office said the prime minister told Obama the deal will allow Iran to acquire nuclear weapons as well as more money to use in menacing Israel.

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Economic effects could be substantial for both Iran and the world.

In trading Tuesday, benchmark U.S. crude oil prices were volatile, falling quickly then rising. Iran is an OPEC member, but its oil production has been affected for years by sanctions over its nuclear program. Any easing of the sanctions could see Iran sell more oil, which could bring down crude prices. That doesn't automatically mean lower gasoline prices, however.

Iran also stands to receive more than $100 billion in assets that have been frozen overseas and an end to various financial restrictions on Iranian banks.

The nearly 100-page accord announced Tuesday aims to keep Iran from producing enough material for an atomic weapon for at least 10 years and impose new provisions for inspections of Iranian facilities, including military sites.

With Obama's final term ending in January 2017, the long-term agreement is sure to be a hot topic for the many candidates vying to take his place.

Kerry: If Iran cheats, "we will know it, because we will be there....and we will be able to respond accordingly." <a href="https://twitter.com/hashtag/cbc?src=hash">#cbc</a> <a href="https://twitter.com/hashtag/irandeal?src=hash">#irandeal</a> —@NahlahAyed

Democrat Hillary Rodham Clinton praised the deal and declared herself ready to enforce it "vigorously, relentlessly" as president. Republican Jeb Bush issued a denunciation, saying the deal only delays the danger and "over time it paves Iran's path to a bomb."

The deal was finalized after more than two weeks of furious diplomacy in Vienna. Negotiators blew through three self-imposed deadlines, with top American and Iranian diplomats both threatening at points to walk away from the talks.

Secretary of State John Kerry, who did most of the bargaining with Iranian Foreign Minister Mohammad Javad Zarif, said persistence paid off. "Believe me, had we been willing to settle for a lesser deal we would have finished this negotiation a long time ago," he told reporters.

The breakthrough came after several key compromises.

Iran agreed to the continuation of a UN arms embargo on the country for up to five more years, though it could end earlier if the International Atomic Energy Agency definitively clears Iran of any current work on nuclear weapons. A similar condition was put on UN restrictions on the transfer of ballistic missile technology to Tehran, which could last for up to eight more years, according to diplomats.

Washington had sought to maintain the ban on Iran importing and exporting weapons, concerned that an Islamic Republic flush with cash from sanctions relief would expand its military assistance for Syrian President Bashar Assad's government, Yemen's Houthi rebels, the Lebanese militant group Hezbollah and other forces opposing America's Mideast allies such as Saudi Arabia and Israel.

Iranian leaders, backed by Russia and China, insisted the embargo had to end as their forces combat regional scourges such as the Islamic State of Iraq and Syria.

Iranian Foreign Minister Mohammad Javad Zarif called the deal a 'historic moment' and one that offers 'a new chapter of hope' for diplomacy in the Middle East. (Leonhard Foeger/Reuters)

Another significant agreement will allow UN inspectors to press for visits to Iranian military sites as part of their monitoring duties, something the country's supreme leader, Ayatollah Ali Khamenei, had long vowed to oppose. However, access isn't guaranteed and could be delayed, a condition that critics of the deal are sure to seize on.

Under the accord, Tehran would have the right to challenge UN requests, and an arbitration board composed of Iran and the six world powers would then decide on the issue. The IAEA also wants the access to complete its long-stymied investigation of past weapons work by Iran, and the U.S. says Iranian co-operation is needed for all economic sanctions to be lifted.

IAEA chief Yukiya Amano said Tuesday his agency and Iran had signed a "roadmap" to resolve outstanding concerns, hopefully by mid-December.

The deal didn't come together easily, as tempers flared and voices were raised during debates over several of the most contentious matters. The mood soured particularly last week after Iran dug in its heels on several points and Kerry threatened to abandon the effort, according to diplomats involved in the talks. They weren't authorized to speak publicly on the private diplomacy and demanded anonymity.