While Iran d id not reject the plan outright, state TV 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.
Iranian opposition to the U.N. 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 U.S., 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," according to state TV.
President Barack 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 U.S. 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 U.N. 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 the U.S. 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 - the U.S., Russia and France - would approve the plan by Friday.
The three countries heeded his call Friday before Iran announced 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.
The country is currently enriching uranium to a 3.5 percent level for a nuclear power plant it is planning to build in southwestern Iran. 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 (1,100 kilograms) 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 2,205 pounds (0.98 tons, 1,000 kilograms) is the commonly accepted amount of low-enriched uranium needed to produce weapons-grade uranium for a single nuclear bomb.
Based on the present Iranian stockpile, the U.S. has estimated that Tehran could produce a nuclear weapon between 2010 and 2015, an assessment that broadly matches those from Israel and other nations.
Associated Press Writers Vladimir Isachenkov in Moscow and Angela Doland in Paris contributed to this report.