The response will come as a disappointment to the United Nations, Russia and France, which endorsed the plan Friday they drafted in discussions with Iran earlier in the week. The agreement was meant to ease Western fears about Iran’s potential to make a nuclear weapon.

While Iran did not reject the plan outright, a source quoted by state television said that Tehran was waiting for a response to its own proposal to buy nuclear fuel rather than ship low-enriched uranium to Russia for further enrichment. Iran has often used counterproposals as a way to draw out nuclear negotiations with the West.

“The Islamic Republic of Iran is waiting for a constructive and confidence building response to the clear proposal of buying fuel for the Tehran research reactor,” state TV quoted an unnamed source close to Iran’s negotiating team as saying Friday.

There was no official comment, and it was unclear whether the source represented the government’s final word on the subject.

Iranian opposition to the United Nations plan could be driven by concerns that it weakens Iran’s control over its stockpiles of nuclear fuel and could be perceived as a concession to the United States, which suspects Iran is using its nuclear program as a way to covertly develop weapons -- an allegation denied by Tehran.

An unnamed member of Iran’s negotiating team urged world powers Friday to “refrain from past mistakes in violating agreements and make efforts to win the trust of the Iranian nation,” state television also reported.

President Obama has stepped up diplomatic engagement with Iran since he took office in January and has faulted the Bush administration for refusing to talk to American adversaries. But he has also threatened harsher sanctions if Iran does not cooperate to ease fears about the nature of its nuclear program.

The U.N. Security Council has already passed three sets of sanctions against Iran for failing to suspend uranium enrichment, but the U.S. faces a serious challenge in convincing Russia and China to go even further because of their close ties to Tehran.

The draft International Atomic Energy Agency agreement was formalized Wednesday after three days of discussions in Vienna. The talks followed a similar meeting at the beginning of October in Geneva that included the highest-level bilateral contact between America and Iran in years.

The head of the U.N. nuclear watchdog, Mohamed ElBaradei, said after the completion of the Vienna talks that he hoped Iran and its three interlocutors — America, Russia and France — - would approve the plan by a Friday deadline.

The three countries heeded his call Friday before Iran appeared to obliquely announce its preference to buy the 20 percent-enriched uranium it needs for its Tehran reactor, which has been producing medical isotopes for the past few decades, rather than export its own uranium to Russia for enrichment.

The country is currently enriching uranium to a 3.5 percent level. Iranian officials have said it is more economical to purchase the more highly-enriched uranium needed for the Tehran reactor than produce it domestically.

The Vienna-brokered plan would have required Iran to send 1.2 tons of low-enriched uranium — around 70 percent of its stockpile — to Russia in one batch by the end of the year, French Foreign Ministry spokesman Bernard Valero said Thursday.

After further enrichment in Russia, France would have converted the uranium into fuel rods that would be returned to Iran for use in the Tehran reactor, he said.

Iran agreeing to ship most of its enriched uranium abroad would significantly ease fears about Tehran’s nuclear program, since just under a ton of low-enriched uranium is the commonly accepted amount needed to produce weapons-grade uranium for a single nuclear bomb.

Based on the present Iranian stockpile, the United States has estimated that Tehran could produce a nuclear weapon between 2010 and 2015, an assessment that broadly matches those from Israel and other nations.