"Frustration is mounting," State Department spokesman Ian Kelly told a news briefing, noting that Tehran had still not made a formal reply to a proposal drafted by the International Atomic Energy Agency (IAEA) more than a month ago.
"We're not prepared to actually pronounce that they have rejected the deal because they haven't formally rejected the deal yet," Kelly said.
"We always hesitate to give a formal deadline - but I would just say that time is very short." Kelly did not specify the time frame he meant by "short."
Kelly added that an IAEA report this week that said Iran's belated revelation of a new uranium enrichment site raised concern about possible further nuclear secrets underscored the need for full Iranian compliance with its international obligations.
The draft deal brokered by the IAEA, the UN nuclear watchdog, calls on Iran to send some 75 percent of its low-enriched uranium (LEU) to Russia and France to be turned into fuel for a Tehran medical research reactor.
Iran, which says its nuclear program is for peaceful purposes and not making bombs, says it prefers to buy reactor fuel from foreign suppliers rather than part with its low-enriched uranium. Such uranium could be used for bombs if enriched further.
Tehran has yet to give a full, official reply on the proposal drafted in September after talks involving Iran, France, Russia and the United States. Iran accepted the Western-backed proposal in principle on October 1.
The United States has rejected Iranian calls for amendments and further talks on the deal, saying it is ready to seek further international sanctions against Tehran should the deal fall through.
"The failure to provide a response to this, and its overall noncompliance as laid out in the IAEA agreement, frankly doesn't give us a whole lot of confidence that they will respond formally," Kelly said.