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 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 the condition of anonymity.

“There has been a very positive line out of Tehran on the talks. 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 Feb. 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.”

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.