Abbas Araqchi however ruled out sending Iran's nuclear stocks abroad, a key demand of world powers, while insisting that all UN Security Council resolutions and sanctions must be lifted.
"Getting to an accord is doable. Solutions have been found for numerous questions. We are still working on two or three issues... The talks are in their final phase and are very difficult," Abbas told reporters in Lausanne.
"We are optimistic, the chances of getting a deal are there. But this requires the other side taking the necessary decisions and demonstrating their political will," he said.
Sending abroad Iran's stockpiles of low-enriched uranium, currently enough for several nuclear weapons if further processed, would make any push by Iran to get the nuclear weapons much more difficult.
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This, combined with slashing the number of uranium enrichment centrifuge machines, would extend the so-called "breakout" time needed by Iran to assemble enough fissile material for a bomb.
Iran denies wanting nuclear weapons, and Araqchi reiterated today, two days before a deadline to agree the contours of a deal, that Tehran wanted sanctions lifted under any accord "under a precise programme".
The powers are prepared to suspend sanctions, not terminate them, and over time to ensure Iran does not violate the possible deal. Doing so with the UN sanctions, however, will be tricky, according to experts.