Flip-Flopping, Excessive Demands Hinder Nuclear Talks: Iran

"Unfortunately, we are witnessing both shifts in the positions and excessive demands, and we are also witnessing that a number of countries in the (Group) 5+1 have different stances, and this has made the situation difficult," Zarif told reporters in Vienna early on Friday.

He said the nuclear negotiations between Iran and the Group 5+1 (Russia, China, the US, Britain, France and Germany) have specific difficulties, particularly during the final days of the talks.

"With a single word added or removed (from the text of a final deal), the final result could undergo fundamental change, that's why we are negotiating carefully and the other side is working carefully as well," Zarif added.

The minister made it clear that Iran will press ahead with the talks and will never leave the negotiating table, as it has never left it before.

"We have now reached a stage that the other sides should really decide whether they seek a deal or pressure. We have repeatedly announced that an agreement and pressure do not go together, and they have to choose one of these two options," the top Iranian negotiator said.

Earlier on Thursday, Zarif broadcast a message of determination to stay at the negotiating table.

"We're working hard, but not rushed, to get the job done. Mark my words; you can't change horses in the middle of a stream," he had tweeted.

Diplomats from Iran and the six powers are in Vienna to hammer out a long-awaited deal over Tehran's peaceful nuclear program.

They have signaled that an extension to a deadline set by the US Congress to complete long-running negotiations would not be fatal to the talks, which have already gone past several previous self-imposed deadlines.

The negotiators have given themselves until the end of the day on Friday. But if a deal is not reached by 6:00 a.m. in Vienna (0400 GMT), the Republican-led US Congress will have 60 days rather than 30 days to review it.