Secretary of State John Kerry says the United States is ready to walk away from the negotiating table if Tehran is unwilling to make the "hard choices" necessary to achieve a deal with the West on limiting its nuclear program.

However, Kerry also said that he and his Iranian counterpart, Mohammad Javad Zarif, were making "genuine progress" on "several of the most difficult issues" that remain to be resolved.

"If hard choices get made in the next couple of days, made quickly, we could get an agreement this week, but if they are not made we will not," he said outside a Vienna hotel where days of intense closed-door meetings involving Iran and the so-called P5+1 countries — the U.K., China, France, Germany, Russia and the United States — have taken place.

French Foreign Minister Laurent Fabius said on Sunday that the time is now for Iran to decide whether it will do what is necessary to resolve outstanding issues.

"The principle question is to see whether Iran will accept to make clear commitments on what has not been clarified. We hope so," Fabius said. "Everything still has to be clarified and France's attitude will be the same as always — constructive firmness."

German Foreign Minister Frank-Walter Steinmeier said what is required to reach a deal is "courage."

Zarif said Friday that he was hopeful that a deal could be done "because I see [the] emergence of reason over illusion."

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