Iran's nuclear negotiator Abbas Araqchi said Thursday that uranium enrichment is non-negotiable principle for his country in ongoing nuclear talks in Geneva.

"The principle of uranium enrichment is non-negotiable, but its details including the level, place and amount of enrichment can be discussed," Araqchi told Iranian reporters live on state TV.

"Uranium enrichment is one of the toughest, most important and most sensitive parts of the talks," he said, adding that "What is obvious is that we will not enter any agreement unless enrichment (for Iran) is mentioned in its very preliminary stage to its end."

U.S. Secretary of State John F. Kerry said Wednesday that " Whatever a country decides or doesn't decide to do (uranium enrichment) or is allowed to do (so) and permitted under the rules depends on a negotiation."

Fresh round of negotiations between world powers and Iran on the country's nuclear issue, which started on Wednesday in Geneva, entered the second day on Thursday.