The International Atomic Energy Agency is hosting talks aimed at getting the US to return to a 2015 deal abandoned under former president Donald Trump and lift sanctions, and to bring Iran back to full compliance with nuclear obligations it retreated from in response.

An agreement reached with the IAEA in February is due to expire later this month, potentially impeding the talks in Vienna.

Iran's foreign ministry spokesman Saeed Khatibzadeh said an option after May 21 could be to extend that date "in case the talks are on the right track and Tehran agrees as well".

"Since we are in no rush to conclude these talks, in addition to not allowing them to drag on... we do not want any date to prevent our negotiating team from precisely carrying out Tehran's instructions," he told reporters.

The "temporary solution" reached in February allowed UN inspectors access to Iran's declared nuclear sites.

But Iran suspended so-called "voluntary transparency measures" -- notably inspections of non-nuclear sites, including military ones suspected of nuclear-related activity.

Tehran also denied the IAEA access to recordings from monitoring equipment that the UN agency installed at its sites to verify its compliance.

The Atomic Energy Organization of Iran said cameras would keep running at the sites but the withheld footage would be deleted if US sanctions are not lifted by the end of the three-month period.

The changes to the monitoring and inspection regime, ordered last year by Iran's conservative-dominated parliament, are part of a series of retaliatory measures Iran has adopted in response to Trump's 2018 withdrawal from the deal.

Iran's chief nuclear negotiator Abbas Araghchi said Friday he hoped the Vienna talks could reach a conclusion "in the shortest time possible".

Araghchi said the Americans had "expressed readiness to lift a large part of their sanctions" but added that they had not yet gone far enough.

Khatibzadeh said the United States had "accepted a major part of what it has to do" but stressed Iran had also called for the lifting of sanctions that were "meant to destroy" the deal when imposed by the previous US adminstration.

"It is no secret that we have serious disagreements in this field," he said.