Khamenei's intervention came yesterday as both the United States and EU confirmed that US Secretary of State John Kerry and EU foreign policy chief Catherine Ashton would meet in the Austrian capital with Iranian Foreign Minister Mohammad Javad Zarif.
Ashton will first hold bilateral talks next Tuesday with Zarif, as is customary ahead of each round of nuclear negotiations, and a three-way meeting is to be held the next day on October 15, her spokesman Michael Mann said.
"Our focus... Is determining whether it's possible to reach an agreement by November 24 that effectively closes down Iran's pathways to nuclear materials for a nuclear weapon," State Department spokeswoman Jen Psaki told reporters.
US officials "continue to believe that there is still adequate time to work through these issues and arrive at a comprehensive agreement that will give the international community assurances that Iran will not acquire a nuclear weapon," she added.
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But talks have stalled over the issue of Iran's future capacity for uranium enrichment and the timetable for the lifting of international sanctions against Tehran.
"We still don't have an understanding of the major issues, so that's what the purpose of the discussion is," Psaki said, adding there would be "many more meetings" including at expert level as they seek to hammer out a deal.
One of the stipulations includes "the absolute need for Iran's uranium enrichment capacity to be 190,000 SWU (Separate Work Units)" -- close to 20 times its current processing ability.
Iranian officials say this is needed to produce fuel for its Bushehr reactor, which is being provided by Russia until 2021.