Iran has just days left to accept a deal on its nuclear program at talks in Vienna, France said on Wednesday, warning that a major crisis would be unleashed if there is no agreement.

"It is not a question of weeks, it is a question of days," French Foreign Minister Jean-Yves Le Drian told the Senate, adding that the "moment of truth" had now arrived in the marathon process.

World powers are entering the final stage of talks with Iran in Vienna aimed at reviving the 2015 deal, which was rendered moribund by the US walking out of the accord in 2018.

The administration of president Donald Trump reimposed sanctions against Iran, battering the Iranian economy, while Tehran ramped up its nuclear work in response in violation of the Joint Comprehensive Plan of Action (JCPOA).

"We need political decisions from the Iranians. They have a very clear choice," Le Drian said.

"Either they unleash a serious crisis in the next days... or they accept an agreement that respects the interests of all the parties, especially those of Iran," he said.

He described a deal as being "within grasp" and noted there was now agreement on an accord between the European powers in the negotiations -- France, Germany and Britain -- as well as China, Russia and the US.

"We have found convergence significant enough to enable an agreement that is within grasp right now," he said.

But he said that time was running out because Iran was continuing to intensify its nuclear activities in violation of the 2015 deal, which aimed to provide safeguards that Tehran would never seek a nuclear weapon.

"The more this goes on, the more Iran is accelerating its nuclear procedures," he said.