US President Barack Obama has lauded a landmark nuclear agreement with Iran as vindication of his diplomatic approach and a chance for a "new direction" in decades of vexed relations with Tehran.
Obama said the deal - which would curb Iran's nuclear program in return for substantial international sanctions relief - cut off "every pathway" to an Iranian atomic weapon.
"Today, because America negotiated from a position of strength and principle, we have stopped the spread of nuclear weapons in this region," he said in a White House address on Tuesday (local time).
Describing a "difficult history" between Iran and the United States that "cannot be ignored," Obama shaped it as a diplomatic victory that showed "it is possible to change."
"This deal offers an opportunity to move in a new direction. We should seize it," he said.
Relations between the US and Iran were smashed amid the tumult of the 1979 Islamic Revolution and the subsequent seizing of hostages at the US embassy in Tehran.
Obama came to office vowing to talk directly to Tehran and to try to reach a negotiated deescalation - a marked shift from his predecessor, who rejected a similar deal struck by European countries.
"This deal demonstrates that American diplomacy can bring real and meaningful change," he said.
But, he warned, if Iran steps back from measures agreed in the lengthy agreement, all sanctions "will snap back into place."
Obama insisted the alternative to diplomacy was more violence in a region already beset by instability.
"Put simply, no deal means a greater chance of more war in the Middle East," he said.
Firmly tethering his presidential legacy to Tuesday's deal, Obama had a further warning to domestic opponents.
He said it was "irresponsible" to walk away from the agreement and vowed to veto any effort in the Republican-controlled Congress to block it.
Diplomats have long warned the deal will not bring smooth waters.
Even if Iran allows intrusive monitoring of nuclear sites, its security services continue to back groups throughout the Middle East that Washington has linked to terror attacks.
Obama said the deal was based on verification, not trust, and noted that differences between the two countries were "real."
Analysts have also warned that Iran's leaders may need to toughen anti-American rhetoric to ensure the backing of regime hardliners angered at the prospect of a deal with a power they view as the "Great Satan."