"The sanctions easing offered at this stage do not deal with those sanctions having the greatest impact ... oil and financial," the U.S. official said Wednesday.

The P5+1 countries have repeatedly said ending the sanctions would be possible only if Iran took "concrete confidence-building steps."

According to the senior U.S. official, Washington's greatest concerns are about Iran's continued enrichment of uranium to a level of 20%, which is a step closer to the 90% uranium enrichment needed for a nuclear bomb.

Also, the U.S. government says it's worried about the Fordo enrichment facility, which is buried deep beneath a mountain near the Iranian holy city of Qom.

On Wednesday, the U.S. official said the current P5+1 proposal calls for Iran to "suspend enrichment at Fordo and constrain the ability to rapidly resume enrichment there."

In his statements to the media, Jalili said the six-nation diplomatic bloc had not requested the closure of Fordo in Almaty.

"There is no justification for its closure, and they did not ask so," Jalili said, speaking through an interpreter.

Western diplomats were careful to characterize this week's talks as "useful."

And at least one longtime observer of Iran and its nuclear program said the latest meeting appeared to have produced some positive results.

"What is new in these Almaty talks are the positive reactions from Iran to the P5+1 proposal and what appear to be substantial adjustments of the P5+1 proposal itself," said Scott Peterson, author of a recently published book on Iran titled "Let the Swords Encircle Me."

Peterson said the apparent changes in the proposal "allow Iran -- at least at this stage -- to continue with its nuclear program, but in a more restricted way."