Iran failed to meet a deadline today for accepting a draft deal on its nuclear program, telling the U.N.'s nuclear watchdog it needed more time to provide a response.
"Iran informed the Director General [Mohamed El Baradei] that it is considering the proposal in depth and in a favorable light, but it needs time until the middle of next week," the U.N. agency said in a statement.
The U.S., France, and Russia have all agreed to the deal, which they had negotiated with Iran earlier this week in Vienna. The pact details plans to export the majority of Iran's low enriched uranium in exchange for more highly enriched nuclear fuel rods. It would have satisfied Western powers by converting Iran's uranium stockpile into a form that is harder to use in building a nuclear weapon.
"It comes as no surprise that Iran is mulling over it and considering ways in which that agreement could be improved on or further refined," said Kaveh Afrasiabi, an Iranian analyst with ties to the government of Iranian President Mahmoud Ahmadinejad. "I'm still optimistic that Iran will agree to a modified version of the agreement."
U.S. State Department spokesman Ian Kelly said, "We are looking for Iran to make concrete steps and there is an agreement in principle to make concrete steps."
The deal was designed to provide fuel for the Tehran Research Reactor, a facility that produces radioisotopes for medical purposes, including cancer treatment.
Experts say the sticking points on the deal, from Iran's perspective, are how much of its uranium would be sent abroad and in how many installments. The original deal called for 1,200 kg, or roughly 80 percent of its stockpile, sent in one shipment.
Iran's Stall on Nuke Deal May Mean More Negotiations Ahead
"It took Iran several years to put that mass together. It can't now send it all abroad," said Afrasiabi, citing concerns from Iran's parliament, or Majles , that exporting uranium in exchange for fuel would set a bad precedent.
At one point Iran hinted it was interested in buying the fuel without sending its own uranium abroad, which would have negated the perceived benefit for Western countries.
Iran's lack of a Friday response does not call off the deal, say U.S. analysts.
"It doesn't look great from Washington that they haven't responded yet, but we're used to this. These are the kinds of hiccups that come up in the course of getting where you want to go," said Jacqueline Shire of the Institute for Science and International Studies. "It's way too soon to start to feel demoralized."
In Iran, press outlets aligned with the ruling conservative camp have portrayed Iran's hard-line negotiating tactics as a victory for the regime, which has been battered by internal disputes over the recent presidential election.
"After the post-election events, the West thought that they could pressurize us to get as many concessions as they could in nuclear negotiations. They thought that Iran had been weakened…however, Iranian negotiators have been able to take part in the talks with determination and impose another defeat on the West," wrote Quds, a hardline newspaper.
Iran insists on its right to continue enriching uranium, despite the objections of Israel and Western powers, and calls from the U.N. Security Council to stop. The draft deal negotiated in Vienna, experts say, implicitly legitimizes Iran's enrichment activities, leading to criticism of the plan by conservatives in the U.S.
Israel expressed dismay over the deal, arguing that it gives the Iranian nuclear ambitions legitimacy, but does not deal what Israel claims is a covert Iranian effort to develop a nuclear weapon.
Even with some degree of bluster, the Iranian government is seen as open to a deal with the West. The Vienna proposal originated as a request from Iran to provide fuel for the reactor, sent to the International Atomic Energy Agency this summer. It has since been developed into a fuel swap now considered as the first major opportunity to resolve Iran's six-year nuclear standoff. The IAEA's El Baradei said he hoped the deal would signal "a new era of cooperation," and eventual normalization of ties between Iran and the West.
Nuclear Inspectors Due to Arrive in Iran This Weekend
IAEA personnel are expected to arrive in Iran this weekend for inspections of the Fardo nuclear enrichment site, a newly-revealed facility near the holy city of Qom. The West and Israel have long questioned Iran's claims that is nuclear program is for peaceful civilian purposes, and whether there are other nuclear sites that remain undeclared.
ABC News' Luis Martinez and Simon McGregor-Wood contributed to this report