A nuclear deal between Iran and world powers was "doable", Tehran's lead negotiator has said in crunch talks in Switzerland, with "two or three" issues still to be resolved.

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 on Sunday.

"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.

"The export of stocks of enriched uranium is not in our program and we do not intend sending them abroad .... There is no question of sending the stocks abroad."

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.

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 on Sunday, two days before a deadline to agree the contours of a deal, that Tehran wanted sanctions lifted under any accord "under a precise program".

"All the sanctions have to be lifted - those of the EU, the United States and the UN Security Council. There are six (UN Security Council) resolutions that have to be annulled," Araqchi said.

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.