"We are going to wait to see what the official response is before we give any kind of possible reaction," said State Department spokesman Ian Kelly.
It would be "unfortunate" if Iran rejected the deal, Kelly said, refusing to rule out changes to the original accord proposed by the five veto-wielding members of the UN security council, plus Germany.
Iranian television reported that Tehran would demand "very important changes" to the plans, which envisage Russia further enriching Iran's low-enriched uranium before returning it for use at a research reactor.
Western officials hope the arrangement would strip the Islamic republic of any need to produce highly-enriched uranium, which they fear could be used as fissile material for a nuclear bomb.
Iranian television said Tehran would give its response to the International Atomic Energy Agency (IAEA) within 48 hours.
"What we're waiting for is a response to the IAEA," Kelly said. "We understand it's supposed to come the middle of this week, and so I assume in the next couple of days we'll get a response.
"We're not going to respond to other press reports and other voices out of Iran. We're doing this as a group. We're not going to respond individually to these proposals," he said, referring to the so-called P5+1 of Britain, China, France, Russia, and the United States plus Germany.
