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 NATO 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 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".

French President Francois Hollande said "the world is making headway" after the deal, while NATO chief Jens Stoltenberg called it a "historic breakthrough" and German Chancellor Angela Merkel hailed it as "an important success" of international diplomacy.

Hundreds of Iranians poured onto the streets of Tehran to celebrate after the Ramadan fast ended at sundown.

"Maybe the economy is going to change, especially for the young people," said Giti, a 42-year-old woman.

- Stringent UN oversight -

The deal limits 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 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 cut 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 rose Tuesday as markets weighed the effects of the breakthrough. US benchmark West Texas Intermediate for August delivery gained 84 cents at $53.04 (48.19 euros) a barrel on the New York Mercantile Exchange.

The agreement is Obama's crowning foreign policy achievement in six years, and the fruit of Rouhani's bid 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, which controls swathes of Syria and Iraq.

- Decades of enmity -

Russia's Foreign Minister Sergei Lavrov said the deal paves the way for a broader coalition to fight IS, a common enemy of the West and Iran.

But erasing decades of hostility will be tough.

The prospect of better US-Iran relations alarms Saudi Arabia, but Riyadh expressed hope on Tuesday for an end to Tehran's "interference" in Syria, Yemen and elsewhere in the region.

In what was seen as a thinly veiled threat of strikes against Iranian nuclear sites, Israeli Prime Minister Benjamin Netanyahu on Tuesday warned: "We did commit to preventing Iran from acquiring nuclear weapons, and this commitment still stands."

Obama's Republican opponents who control Congress will have 60 days to review the agreement, during which time he cannot waive Congressional sanctions.

The opponents, backed by legions of lobbyists, are set to launch a campaign to try to scupper the deal.

House Speaker John Boehner, a Republican, said Tuesday the agreement was "likely to fuel a nuclear arms race around the world".

Obama will hold a press conference on Wednesday to convince Americans, allies and sceptics about the benefits of the deal.

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 before the UN, US and EU lift their sanctions.

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