The United States expressed mild disappointment Friday that Iran withheld a decision on whether to accept a U.N.-coordinated plan that could ease fears about Iran's potential for making a nuclear weapon.

The U.S., along with Russia and France, officially endorsed the plan Friday. The State Department said it was unhappy that Iran was not ready to embrace the plan, which calls for Iran to ship most of its low-enriched uranium to Russia for further enrichment. The resulting fuel is to be used for a research reactor in Tehran that makes medical isotopes and is under regular monitoring by a U.N. agency.

The plan is attractive from the U.S. point of view because it would consume a large proportion of Iran's stockpile of low-enriched uranium, thereby limiting the potential for it to secretly convert it into uranium suitable for making a nuclear weapon. Iran denies it has any intention of making a nuclear weapon.

The International Atomic Energy Agency, the U.N. nuclear watchdog that is coordinating the uranium shipment plan, said Iran gave notification on Friday that it could not yet provide a final answer.

Iran said it was "considering the proposal in depth and in a favorable light, but needs until the middle of next week to provide a response," the Vienna-based nuclear agency said.

Iranian state television reported that Tehran prefers to simply buy a quantity of higher-enriched uranium from abroad for use in the Tehran reactor, rather than give up a large portion of its own stockpile of low-enriched uranium.

State Department spokesman Ian C. Kelly said the U.S. still hopes Iran will go along with the IAEA option.

"This is a real opportunity for Iran to help address some of the real concerns of the international community about its nuclear program and at the same time still provide for the humanitarian needs of the Iranian people," Kelly said.

"We hope that they will next week provide a positive response," Kelly added. "Obviously we would have preferred to have had a response today. We approach this with a sense of urgency. The international community's been waiting a long time for Iran to address some of our real concerns about their intentions."

The draft proposal to ship Iran's uranium to Russia was put forth earlier in the week after three days of talks in Vienna.