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 that 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 said.

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, political decision," Kerry said.

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 year that gives us the opportunity if they were to try to do that," Kerry added.

And he repeated US insistence that "no deal is better than a bad deal."

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.

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