VIENNA -- Both the United States and Iran on Tuesday voiced their cautious view of the Iranian nuclear talks in Vienna, saying the talk could be very difficult, as the significant gaps remain unresolved.

Speaking to reporters after an unofficial dinner with European Union foreign policy chief Catherine Ashton ahead of the plenary session of the nuclear talks, Iranian Foreign Minister Mohammad Javad Zarif said the talks could be difficult, but he believes there is hope for the comprehensive deal.

"This is going to be a difficult and time consuming exercise," Zarif said. "I believe everybody hopes that we can achieve this within a reasonable period of time."

A senior U.S. official on Tuesday made comments over the upcoming talk anonymously, noting the talk could be very difficult, due to the remaining significant gaps between Iran and the west.

A fresh round of nuclear talks between Iran and five permanent members of the United Nations Security Council plus Germany, or the P5+1 group, was scheduled for May 13 in Vienna.

The talks aim to draft a comprehensive agreement that allows Iran to continue its nuclear program while reassuring the powers it will not be used for military purposes.

Iran and the P5+1 clinched an interim deal in Geneva last November, in which Iran agreed to cap its nuclear program in exchange for easing some financial sanctions against it. The Geneva deal took effect on Jan. 20.

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