Progress in this week's nuclear talks between Iran and six major powers depends on how Tehran responds to a proposal offered by the six in February, a senior U.S. official said on Wednesday.

"How far we get ... depends on what the Iranians come back with in terms of a response on the substance to our proposal," said the official, who spoke on condition of anonymity."There has been a very positive line out of Tehran on the talks so far. We hope that that positive talk will now be matched with some concrete responses and actions on the Iranian side," the official added.

The United States and its allies suspect Iran of using its civilian nuclear program as a cover to develop atomic weapons. Iran denies this, saying its program is entirely peaceful.At February 26-27 talks with Iran in Almaty, Kazakhstan, the six major powers - Britain, China, France, Germany, Russia and the United States - offered modest sanctions relief in return for Iran curbing its most sensitive nuclear work.

The senior U.S. official put the onus on Iran to put forward a substantive response at Friday's talks in Kazakhstan, known informally as "Almaty II," but also played down the idea that this week's discussions constituted a last chance for Iran."I would hope that we're not at any last chance," said the official. "If we are not sure about how much we've gotten and whether we have gotten enough, we'll go back and consult with capitals before we reach any ultimate conclusion here.

"So I think we have time and space to consider what we hear," the official added. "We hope that they make concrete, substantive and specific responses so that we can go to work."In February, Western officials said the offer presented then by the six powers included an easing of a ban on trade in gold and other precious metals, and a relaxation of an import embargo on Iranian petrochemical products. They offered no details.

In exchange, a senior U.S. official said, Iran would among other things have to suspend uranium enrichment to a fissile concentration of 20 percent at its Fordow underground facility and "constrain the ability to quickly resume operations there."This appeared to be a softening of a previous demand that Iran ship out its entire stockpile of higher-grade enriched uranium, which it says it needs to produce medical isotopes.

Iran says it has a sovereign right to enrich uranium for peaceful purposes, and wants to fuel nuclear power plants so it can export more oil.But 20-percent purity is far higher than that needed for nuclear power, and rings alarm bells abroad because it is only a short technical step away from weapons-grade uranium.Asked whether the United States wanted an absolute shut-down of Fordow, the U.S. official declined to provide details but suggested some flexibility.

"I won't be able to give you specifics on Fordow, except to say that our objective to deal with Fordow remains the same objective," said the official. "There are many ways to get there and our proposal is one vehicle for doing that, but our desire to make sure that Fordow does not remain the concern that it is very much part of the proposal ... we have put on the table."