Obama pledges to veto any congressional challenge to Iran accord

US president Barack Obama has proclaimed a landmark nuclear deal with Iran, the conclusion of 12 years of negotiations, as a "meaningful change" that makes the US and the world "safer and more secure".

Speaking just hours after the US and five other world powers reached an agreement with Iran in Vienna, preventing Tehran from building an atomic bomb in return for lifting economic sanctions, Mr Obama said that the deal meant that “every pathway to a nuclear is cut off”. “Put simply, no deal means a greater chance of more war in the Middle East,” he said.

As the agreement heads to Congress for a 60-day review, the president said, during an unusual 7am announcement, that he welcomed a robust debate on the agreement with politicians on Capitol Hill.

He was confident that the agreement would meet national security interests of the US and its allies. However, he promised to “veto any legislation that prevents the successful implement of this deal”.

The president added: “We do not have to accept an inevitable spiral into conflict. We certainly shouldn’t seek it. And precisely because the stakes are so high, this is not the time for politics or posturing.”

Iran had a stockpile of nuclear material that could produce up to 10 nuclear weapons, Mr Obama said, but after the deal, that would “be reduced to a fraction of what will be required for a single weapon”.

This, for most critics, is unacceptable as they refuse to accept anything short of a full dismantlement of a nuclear programme controlled by a country that has been an adversary of the US for 35 years.

Opponents must find a two-thirds majority in both houses of Congress – 290 in the House of Representatives and 67 in the Senate – to derail the agreement. Even with Democratic rebels opposed to the Iran deal, these will be extremely high hurdles to overcome.

Republican house speaker John Boehner called the deal “unacceptable”, saying it was “going to hand a dangerous regime billions of dollars of sanctions relief while paving the way for a nuclear Iran”.

Hillary Clinton, the 2016 Democratic presidential frontrunner, cautiously supported the deal, striking a balance that a campaign requires of backing a president she served as secretary of state and warning that she would keep fighting Iranian-backed terror groups. Visiting the US Capitol, Ms Clinton called the deal "an important step" but stressed that she would evaluate the agreement carefully and ensure that it was "enforced vigorously, relentlessly".

“This does put a lid on the nuclear programme, but we sill have a lot of concern about the bad behaviour and the actions by Iran, which remains the largest state sponsor of terrorism,” she said.

Republican candidates were unanimous in their opposition to the agreement which was brokered by the Democratic administration. Former Florida governor Jeb Bush, the brother of former US president George W Bush and the Republican frontrunner for 2016, called the deal "dangerous, deeply flawed and short-sighted".

Scott Walker, the Wisconsin governor and the most recent candidate to enter the 2016 race for the White House just hours before the Iran deal was announced, said that it "will be remembered as one of America's worst diplomatic failures".

Florida senator Marco Rubio said the deal undermined American national security and he expected a significant majority in Congress would share his scepticism and “vote it down”.

"Failure by the president to obtain congressional support will tell the Iranians and the world that this is Barack Obama's deal, not an agreement with lasting support from the United States, " he said.