(JTA) — Talks aimed at drafting an agreement between Iran and six world powers on the Iranian nuclear program ended with large gaps remaining between the two sides.

Iran has been negotiating with the United States, the United Kingdom, France, Germany, Russia and China since last year toward an accord that would lift international sanctions against Iran in exchange for Iran curbing its nuclear program. The sides have set a July 20 deadline for the talks, and hoped to draft an agreement on Saturday.

No draft emerged, according to AFP, because Iran refused to stop or convert its heavy-water reactor in the city of Arak, which can be used to make nuclear weapons-grade plutonium. Iran is widely believed to be seeking nuclear weapons, though it insists the program is for civilian purposes only.

“This has, candidly, been a very slow and difficult process, and we are concerned with the short amount of time that is left,” a senior US official said. “In any negotiation there are good days and bad days, and there are ups and downs. This has been a moment of great difficulty, but one that was not entirely unexpected.”

No date has been set for the next round of negotiations.

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