Major powers clinched a historic deal Tuesday aimed at ensuring Iran does not obtain the nuclear bomb, opening up Tehran's stricken economy and potentially ending decades of bad blood with the West.

Reached on day 18 of marathon talks in Vienna, the accord is aimed at resolving a 13-year standoff over Iran's nuclear ambitions after repeated diplomatic failures and threats of military action.

It was hailed by Iran, the United States, the European Union and others but branded a "historic mistake" by Tehran's archfoe Israel.

US President Barack Obama said the accord meant "every pathway to a nuclear weapon is cut off."

"This deal offers an opportunity to move in a new direction. We should seize it," he said in an address to the nation.

He vowed to veto any Congressional effort to block the deal, reached between Tehran and the United States, Russia, China, Britain, France and Germany.

Underscoring the tectonic shift in relations, Iranian state television broadcast Obama's statement live, only the second such occasion since the 1979 Islamic revolution.

Iranian President Hassan Rouhani said in his own live televised address that "God has accepted the nation's prayers" and that the accord would lift "inhumane and tyrannical sanctions".

"Iran will never seek a nuclear weapon, with or without the implementation" of the Vienna deal, he added.

EU foreign policy chief Federica Mogherini described the agreement as "a sign of hope", while Russian President Vladimir Putin said the world had "breathed a huge sigh of relief."

Syrian President Bashar al-Assad, a close ally of Iran, called the agreement a "great victory".

The deal was also welcomed on the streets of Tehran.

"It's great news because the economy will boom," Iranian accountant Behnam Arian said at Argentine Square, a busy commercial district.

- Stringent UN oversight -

The deal puts strict limits on Iran's nuclear activities for at least a decade and calls for stringent UN oversight, with world powers hoping this will make any dash to make an atomic bomb virtually impossible.

In return Iran will get sanctions relief although the measures can "snap back" into place if there are any violations.

The international arms embargo against Iran will remain for five years with deliveries only possible during that time with permission from the UN Security Council, diplomats said.

Tehran has accepted allowing the UN nuclear watchdog tightly-controlled "managed access" to military bases, an Iranian official said.

Iran will slash by around two-thirds the number of centrifuges, which can make fuel for nuclear power but also the core of a nuclear bomb, from around 19,000 to 6,104.

Painful international sanctions that have slashed the oil exports of OPEC's fifth-largest producer by a quarter and choked its economy will be lifted and billions of dollars in frozen assets unblocked.

World oil prices fell Tuesday following the breakthrough, with US benchmark West Texas Intermediate for August delivery down $1.05 (0.95 euros) at $51.15 (46.46 euros).

The accord -- which was built on a framework first hammered out in April -- is Obama's crowning foreign policy achievement six years after he told Iran's leaders that if they "unclench their fist, they will find an extended hand from us."

It also the fruit of Rouhani's attempts since his election in 2013 to end Iran's isolation.

The agreement may lead to more cooperation between Tehran and Washington at a particularly explosive time in the Middle East with the emergence last year of the Islamic State group, a common enemy, which controls swathes of Syria and Iraq.

- Decades of enmity -

Erasing decades of hostility will be tough, though, as seen in Iranian supreme leader Ayatollah Ali Khamenei's July 11 comments about US "arrogance" and the burning of US and Israeli flags last week.

The prospect of better US-Iran relations alarms Saudi Arabia and other Sunni-ruled Gulf Arab states, which are deeply suspicious of Shiite Iran and accuse it of stoking unrest in Syria, Yemen and elsewhere.

Reflecting the concerns, a senior US official said Obama would soon speak to Israeli and Saudi leaders.

Israeli Prime Minister Benjamin Netanyahu on Tuesday called the deal "a historic mistake for the world."

"We will always defend ourselves," he added.

"We did commit to preventing Iran from acquiring nuclear weapons, and this commitment still stands," he added in what was seen as a thinly veiled threat of pre-emptive strikes against Iranian nuclear sites.

Obama's Republican opponents who control Congress will have 60 days to review the agreement.

During this time Obama cannot waive Congressional sanctions, which for Iran are the most painful.

The opponents, backed by legions of lobbyists, are set to launch an intense campaign to try and secure a two-thirds majority to override a presidential veto and scupper the deal.

House Speaker John Boehner, a Republican, said Tuesday the agreement was "likely to fuel a nuclear arms race around the world", adding that lawmakers would "review every detail of this agreement very closely."

Even if the agreement gets past Congress -- the Iranian parliament and the UN Security Council also have to approve it -- implementing the accord could be a rough ride.

France said it expected Security Council approval "within days", while a US official said a resolution could be introduced next week.

The UN nuclear watchdog will have to verify that Iran does indeed scale down its facilities, clearing the way for the complex choreography of ending UN, US and EU sanctions.