Senior US State Department official says negotiators have made significant progress regarding Iran nuclear deal and it's in a better position, but added some serious issues still remains.

Iran has repeatedly called for guarantees from the US administration that there will be no repeat of Trump's 2018 walkout during Vienna talks.
Iran has repeatedly called for guarantees from the US administration that there will be no repeat of Trump's 2018 walkout during Vienna talks. (AA)

Negotiators have made significant progress in the last week or so on an agreement to revive the 2015 Iran nuclear deal but very tough issues remain, a senior US State Department official has told reporters.

"There's been significant progress over the last week or two. We have significantly narrowed the areas that still need to be resolved. And so, in that sense, there has been progress that has been moving towards a potential deal," he said on Friday.

 "So we are in a better position than we have been. But at the same time, it's important to note that very serious issues remain."

Mohammad Eslami, head of the Atomic Energy Organisation of Iran, said Iran can resume production of highly enriched uranium "at any moment" if the US again pulls out of a nuclear deal with major powers.

He was speaking as talks in Vienna on bringing Washington back into the 2015 agreement abandoned by former US President Donald Trump reach a critical stage.

READ MORE: Iran: West needs to 'take certain decisions' to reach a nuclear deal

Calling for guarantees

During the negotiations in Vienna, Iran has repeatedly called for guarantees from the Biden administration that there will be no repeat of Trump's 2018 walkout, something that partisan divisions on Iran policy have effectively precluded.

The 2015 deal set a 3.67 percent limit for Iran's uranium enrichment, sufficient for its power generation needs although not for some other civilian uses, such as the production of medical isotopes.

Since Trump abandoned the agreement and reimposed crippling sanctions, Iran has responded by producing uranium enriched to 60 percent, a level that has sparked Western concern although it still falls short of weapons-grade.

READ MORE: Iran lawmakers demand guarantees US won't leave restored nuclear deal

Source: TRTWorld and agencies