"This first step, I want to emphasise, actually rolls back the programme from where it is today, enlarges the breakout time, which would not have occurred unless this agreement existed. It will make our partners in the region safer. It will make our ally Israel safer," Kerry told reporters.
Ahead of the deal, Israel had issued repeated broadsides, warning that any easing of sanctions against Iran could enable a "breakout" -- a final leap towards acquiring atomic weapons.
Israeli Prime Minister Benjamin Netanyahu -- whose country is widely assumed to be the Middle East's only atomic-armed nation -- has refused to rule out military action against arch rival Iran to halt its nuclear programme in its tracks.
Netanyahu has also warned the West against being hoodwinked by Iran's apparent newfound appetite for rapprochement with the West since President Hassan Rouhani, himself a former nuclear negotiator and seen as a relative moderate, replaced hardliner Mahmoud Ahmadinejad in August.
Rouhani has vowed to win an easing of the international sanctions that have crippled the Iranian economy.
Kerry said Netanyahu -- "a friend of mine" -- had been kept abreast of the state of play in the talks, which kicked off Wednesday.
"I talk to him several times a week," he said. "I talked to him in the last day about this very issue."
Any differences between the United States and Israel on the issue were simply a matter of "judgement" and "calculation", Kerry insisted.
"There is no difference whatsoever between the US and Israel of what the end goal is -- that Iran will not have a nuclear weapon," he added.
But in a first reaction Netanyahu's office said the nuclear deal was "bad" as Tehran had obtained "what it wanted: the partial lifting of sanctions while maintaining an essential part of its nuclear programme".
European Union foreign policy chief Catherine Ashton, who chaired the world powers' negotiating team in the talks, said Israeli concerns were well understood.
"We talk all the time to Israel. Israel is an extremely important country, and its views are very important to us, and we believe that the security of the people of the region and indeed, you might say, of the world, is addressed best when we can find diplomatic solutions to problems and comprehensive agreements that address concerns," Ashton told AFP.
"I think President Rouhani and his team have made significant moves to seek to address this, to do so in the real spirit of ensuring that they take this forward," she added.
Iran insists that its nuclear programme is peaceful, and rejects Western claims that its uranium enrichment shows it is on a thinly-disguised quest for atomic weapons.
Throwing the ball into Tehran's court, Kerry said it should be a simple matter to prove its critics wrong.
"It ought to be really easy to do the things that other nations do that enrich," he said. "To prove it."