The team has been holding talks in Vienna with representatives of the other parties to a 2015 nuclear accord on how to reintegrate Washington into the deal and to end crippling sanctions and Iran's countermeasures.

US President Joe Biden has said he is ready to reverse the 2018 decision of his predecessor Donald Trump to withdraw from the agreement and reimpose unilateral sanctions.

Tehran's official position, announced by supreme leader Ayatollah Ali Khamenei, is to fall back in line with obligations it suspended only after verifying that "all sanctions" are "truly lifted".

"Verification means that, for instance, the Islamic republic must be able to sign its oil contracts and export it and bring in its revenues via banking channels or consider other uses for it," said Kazem Gharibabadi, Iran's ambassador to the UN's nuclear watchdog and a delegation member.

"Or, in the banking sector, (Iran must) be able to conduct its financial transactions using different financial channels," he said, quoted by Khamenei's official website.

Gharibabadi stressed it was "the only way we can make sure that sanctions have been lifted in action" and not only "on paper".

Iran demands the lifting of all US sanctions, including those reimposed by Trump, "new sanctions" by the administration, and "sanctions imposed using non-nuclear excuses", he said.

Trump blacklisted dozens of Iranian entities in addition to blocking Iran's oil exports and virtually all banking ties -- sanctions that had been lifted in 2016 under the accord curbing its nuclear programme.

According to Gharibabadi, Iran is exploring options in the Vienna talks to prevent a potential repeat of what happened in 2018.

"It is being discussed what should be done regarding Iran's technical commitments, and how they be carried out, in case a party reneges on their promise."