As the clock ticks down to a November 24 deadline for an agreement on reining in Iran's nuclear program in exchange for sanctions relief, Kerry vowed global powers were going to be "very careful, everything will be based on expert advice."
"Whether Iran can make the tough decisions that it needs to make will be determined in the next weeks," Kerry told a forum hosted by The Atlantic magazine.
He refused to give any odds on whether the "critical" deal would be reached, but he warned that any pact must be based on fact and science.
"This must not become an ideological, or a political decision," Kerry said.
The United States has pledged to shut all of Iran's pathways to a nuclear bomb "sufficient that we know we have a breakout time of a minimum of a year that gives us the opportunity to respond if they were to try to do that," Kerry added.
And he repeated the US insistence that "no deal is better than a bad deal."
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While technical experts from Iran and the group of world powers known as the P5+1 have continued to work behind the scenes, no new date has yet been set for the next high-level talks between the political directors and ministers of the seven countries involved.
Kerry hosted a dinner for the EU's foreign policy chief Cathy Ashton late Wednesday as she steps down from her role, after having shepherded the P5+1 negotiations for years.
With all eyes on the diplomatic front, Kerry was also meeting Thursday with the head of the UN atomic watchdog Yukiya Amano.
All sides have repeatedly stressed that serious gaps still remain in the negotiations.
In a sign of the battles still to be resolved, a top Iranian official said Iran wants all Western sanctions to be lifted as part of a deal on its contested nuclear program.
The chairman of the Iranian parliament's National Security and Foreign Policy Commission Alaeddin Boroujerdi rejected as "unacceptable" the US proposal of a gradual lifting of sanctions.
"If we want a definitive accord on November 24, there must be an immediate lifting of sanctions," he told reporters in Paris.
Washington has said that under any deal, EU, US and UN sanctions would only be suspended initially -- meaning they could be swiftly re-imposed -- until Iran proved it had abandoned any moves toward a nuclear weapon.
"If we could take this moment of history and change this dynamic, the world would be a lot safer, and we'd avoid a huge arms race in the region where Saudis, Emiratis, Egyptians, others may decide that if they're moving towards a bomb, they've got to move there too," Kerry added.