"A cautious assessment is that, from the discussions that have taken place in Muscat to date, at least what we have been told is that the US position on the Iranian nuclear issue has moved towards a more realistic one," foreign ministry spokesman Esmaeil Baqaei said, according to the official IRNA news agency.

He added that Iran's "inalienable rights under the Non-Proliferation Treaty (NPT) were recognised", referring to the "peaceful uses of nuclear energy, including enrichment".

Tehran and Washington are due on Tuesday to hold a second round of negotiations -- which began this month -- after earlier talks collapsed during last year's Iran-Israel war.

In June, Israel launched an unprecedented bombing campaign against Iran, triggering a 12-day war during which Israel and the United States struck Iranian nuclear facilities.

Ahead of the latest talks, Baqaei said Iran would press for the lifting of long-standing US sanctions that have deepened the country's economic crisis.

"Time is of the essence for us. Our people are under the pressure of oppressive sanctions, and reason and logic demand that we lift these sanctions as soon as possible," he said.

Key sticking points include Iran's stockpile of enriched uranium, which had reached 60 percent purity before the war and which Washington has repeatedly called on Tehran to reduce to zero.

On Sunday, Israel also called for Iran to have "no enrichment capability" and for all of its enriched material to be transferred abroad.

Considerable uncertainty surrounds the fate of Iran's stockpile of more than 400 kilogrammes of 60-percent enriched uranium that was last seen by watchdog inspectors in June before the war.

Iran has said its enriched material remains under the rubble of nuclear sites struck during the conflict.

Tehran insists on its right to enrichment but has said it is prepared to dilute its highly enriched uranium if all sanctions are lifted.

Dilution involves blending enriched uranium with other material to lower its concentration below a specified threshold.

The latest talks come amid heightened tensions after Washington deployed an aircraft carrier group to the Gulf following Iran's deadly crackdown last month on anti-government protests, which were triggered by economic hardship.