VIENNA - Hours after major powers agreed a deal over Iran’s nuclear programme, the White House on Wednesday launched a campaign to stop sceptics at home and abroad from derailing the long-awaited accord.

The agreement, signed on Tuesday after 18 days of marathon talks in Vienna, aims to ensure Tehran cannot create a nuclear bomb in return for lifting biting sanctions that have crippled Iran’s economy.

It was hailed by the United States, the European Union, Iran and NATO - all of whom hope the deal will end decades of bad blood between the Middle East’s major Shiite Muslim power and the West - 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. Hours after the deal was signed in Vienna the US had already begun its diplomatic offensive in the United Nations, where its diplomats were readying a draft resolution setting out timelines.

The document, expected in the coming days, would also replace the existing framework of Security Council sanctions with the restrictions agreed during negotiations in Vienna, US Ambassador Samantha Power said.

In Washington, Obama faces a challenge from Republicans who control Congress, who have said they will reject the deal as it gives Tehran too much room to manoeuvre and does not safeguard American security interests.

House Speaker John Boehner, a Republican, said Tuesday the agreement was “likely to fuel a nuclear arms race around the world”. Congress has 60 days to review the deal reached between Tehran and the United States, Russia, China, Britain, France and Germany to end a more than 13-year standoff, but the president has vowed to veto any attempt to block it. Obama will hold a press conference on Wednesday to convince Americans, allies and sceptics about the benefits of the deal. 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.

On the streets of Tehran, Iranians festooned their cars with balloons and danced on the street in celebration. “I was thinking about leaving, but now I will stay to see what happens,” said 42-year-old computer programmer Giti.

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 with permission from the UN Security Council, diplomats said.

Tehran has also 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 - and 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.

Oil prices rose early in Asia after news of the deal, with US benchmark West Texas Intermediate for August delivery up 24 cents to $53.28 and Brent crude for August added 25 cents to $58.76.

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.

Washington hopes it may lead to more cooperation with Tehran at a particularly explosive time in the Middle East after the Islamic State group rose to power last year, seizing vast swathes of Syria and Iraq. But it has alarmed some of America’s most important Middle East allies, including major Sunni regional power Saudi Arabia and Israel.

Riyadh said it hoped the deal would end Tehran’s “interference” in Syria and Yemen - where the kingdom is leading a coalition fighting Iran-backed Shiite Huthi rebels - an argument echoed by other Gulf nations.

“Iran could play a (significant) role in the region if it revises its policy and stops interfering in the internal affairs of countries like Iraq, Syria, Lebanon and Yemen,” said an official from the United Arab Emirates.

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

Even politicians from Obama’s own Democratic party were keen to show their tough stance, with presidential contender Hillary Clinton vowing Tehran will never be able to acquire atomic weapons if she is elected to the White House. “As president, I would use every tool in our arsenal to compel rigorous Iranian compliance,” Clinton said in a statement.

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.

The UN nuclear watchdog will have to verify that Iran has indeed scaled down its facilities before the UN, US and EU lift their sanctions.