Iran has significantly slowed the pace at which it is accumulating near-weapons-grade enriched uranium, The Wall Steel Journal reported, quoting sources.
It has also diluted some of its stockpile, the report said, moves that may ease tensions with the US and allow the resumption of nuclear talks.
The report follows the release of four Iranian-American prisoners by authorities in Tehran from Evin prison into house arrest on August 10.
A fifth American, a woman whose detention was recently made public, is under house arrest.
White House National Security Council spokesman John Kirby said he could not confirm the WSJ report but “any steps that Iran might take to slow down enrichment certainly would be welcome.”
“We're not in active negotiations about the nuclear programme,” he said.
The White House said on August 11 there would be restrictions on what Iran could do with any funds unfrozen under an emerging agreement that has led to the release of the Americans.
An estimated $6 billion in Iranian assets are now held in South Korea.
Mr Kirby told reporters that the US would have “full visibility” into where any released Iranian funds are directed and used.
The five Americans will be allowed to leave Iran after the funds are unfrozen, Reuters reported, quoting a source.
The transfer has drawn criticism that US President Joe Biden is effectively paying a ransom for the US citizens and also that allowing Iran to use the money for humanitarian goods could free up funds for its nuclear programme or to back militias in nations such as Iraq, Lebanon and Yemen, Reuters reported.
“We are not in talks with Iran on the Joint Comprehensive Plan of Action [JCPOA],” he said when asked about other US-Iran files being addressed through mediators.
In June this year, Hasan Alhasan, Middle East specialist at the International Institute for Strategic Studies, told The National that Iran appeared close to developing uranium enrichment to within the “threshold” of making a nuclear bomb to “blackmail” the region.
Iran is likely to have enriched its uranium to 80 per cent – 10 per cent away from making it viable for a nuclear device, he claimed.