"No fuel is supposed to leave Iran," the Mehr news agency quoted the deputy secretary of the Supreme National Security Council, Ali Bagheri, as saying.
The shipping out of more than 70 percent of Iran's low enriched uranium stocks is a central plank of the proposed deal with the major powers under which it would receive fuel enriched to the 20 percent level required for a Tehran research reactor from Russia and France.
Bagheri is number two to Saeed Jalili, who heads the council and is Iran's chief nuclear negotiator.
He said Iran was only ready to agree to an exchange inside the country and said it needed to receive the fuel for its reactor at the same time as it handed over its stocks.
"One of the ways to guarantee the supply of fuel for the Tehran reactor is the simultaneous exchange of the 3.5 percent fuel for the 20 percent inside Iran," he said.
He said world powers "know Iran's position and, if they wanted to disagree with a simultaneous exchange they would have done it already."
The head of the UN nuclear watchdog which drafted the deal between Iran and the major powers has said publicly that any simultaneous exchange is unacceptable to the Western powers.
A simultaneous exchange "would not defuse the crisis, and the whole idea is to defuse the crisis," International Atomic Energy Agency chief Mohamed ElBaradei said in an interview with the New York Times earlier this month.
Western governments strongly support the existing plan as they believe it would leave Iran with insufficient stocks of low enriched uranium with which to make an atomic bomb.
They have expressed fears that Iran might otherwise covertly divert some of the stocks and enrich them further to the much higher levels required for a bomb, an ambition Iranian officials strongly deny.
The major powers expressed disappointment on Friday that Iran has "not responded positively" to the UN-drafted proposals.
But the United States said it hoped Iran would still accept the deal.
State Department deputy spokesman Robert Wood said that the United States and the other five powers were not yet "at the point" of closing the window on dialogue.
