Iran's top diplomat on Thursday demanded that the UN nuclear watchdog drop the issue of three undeclared sites, as momentum builds to revive a 2015 nuclear deal.

Washington has said the United States remains adamant that Iran cooperate with the International Atomic Energy Agency to clear up suspicions about earlier work at three undeclared sites.

In June, the IAEA's board of governors adopted a resolution censuring Iran for failing to adequately explain the previous discovery of traces of enriched uranium at three sites which Tehran had not declared as having hosted nuclear activities.

"We are very serious about safeguard issues, and do not want to allow some of the IAEA's baseless accusations to remain," Foreign Minister Hossein Amir-Abdollahian said, state news agency IRNA reported.

The issue has poisoned relations between the IAEA and the Islamic republic, which considers the matter "political in nature, and which should not be used as a pretext to punish Iran", an Iranian diplomat said, quoted by IRNA.

The comments come a day after the United States responded to Iran's proposals on reviving the landmark agreement trashed by former president Donald Trump.

"We are in the process of examining the response of the Americans," Amir-Abdollahian said.

Just weeks after the deal looked dead, the European Union put forward on August 8 what it called a final text to restore the agreement, in which Iran would see sanctions relief and be able to sell its oil again in return for severe limits on its nuclear program.

Iran came back last week with a series of proposed changes, to which the United States formally responded on Wednesday, a day after Tehran accused its arch-enemy of stonewalling.

The 2015 agreement between Iran and six world powers -- Britain, China, France, Germany, Russia and the United States -- gave the Islamic republic sanctions relief in exchange for curbs on its nuclear programme.

The deal formally known as the Joint Comprehensive Plan of Action, or JCPOA, was designed to prevent Iran from developing a nuclear weapon -- something it has always denied wanting to do.

On Tuesday, a senior US official said that Iran had lifted its block on certain IAEA inspections.