The last-ditch talks between between Tehran and the P5+1 group of world powers will likely continue through the weekend, Iran's Foreign Minister Mohammad Javad Zarif was cited in media reports as saying.
Responding to the question on whether a deal could be achieved on Friday, Zarif said he "doubts it", Press TV reported.
"We're making progress. It seems we are staying here for the weekend. We're working," he said, adding, "We're not there yet".
The comments came shortly after the Iranian foreign minister said the P5+1 countries are shifting their positions and pressing excessive demands.
"Unfortunately, we are witnessing both shifts in positions and excessive demands... Several countries (which are members) in the P5+1 group hold different positions. Such issues have made the negotiations difficult.
"We want a dignified agreement and negotiations, and we will continue to negotiate. The deal would be within reach should the other side seek an honorable and balanced accord," he said late on Thursday.
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Zarif, in an article published on Wednesday, reminded the five permanent members of the UN Security Council -- Britain, China, France, Russia, the US plus Germany -- to choose between coercion and agreement, warning against opting for the former.
Iran and the P5+1 countries have been engaged in crucial talks in the Austrian capital Vienna towards reaching a landmark deal over Tehran's nuclear programme.