Iran's Supreme Leader Ayatollah Ali Khamenei has said that the United States had created the "myth" of nuclear weapons to portray Iran as a threat.
Mr Khamenei has supported the talks but has continued to express deep mistrust of the US.
As the highest authority in Iran, the withdrawal of his support could cause the negotiations to collapse.
Iran and six world powers including the US reached a framework accord on Iran's disputed nuclear programme this month.
They will resume negotiations in Vienna this week aiming to reach a final deal by the end of June.
Despite significant progress, the two sides still disagree on several issues.
The issues include how quickly international sanctions would be lifted under a final deal.
