Officials from world powers and Iran were meeting in the Austrian capital for the first time since March, when negotiations -- which began in 2021 to reintegrate the United States into the agreement -- stalled.

"Today's talks in Vienna do not mark a new round of negotiations. These are technical discussions," the three countries -- known as the E3 group -- said in a statement.

"The text is on the table. There will be no re-opening of negotiations. Iran must now decide to conclude the deal while this is still possible. We urge Iran not to make unrealistic demands outside the scope of the JCPoA", or Joint Comprehensive Plan of Action, the statement said.

Britain, China, France, Germany, Iran, Russia and the United States signed the JCPOA in July 2015.

But following the unilateral withdrawal of the United States in 2018 under former president Donald Trump and the re-imposition of US sanctions, Tehran has backtracked on its obligations to curtail its atomic activities, such as uranium enrichment.

The UN's nuclear watchdog, the International Atomic Energy Agency, has found that Iran subsequently exceeded the agreed enrichment rate of 3.67 percent, rising to 20 percent in early 2021.

It then crossed an unprecedented 60-percent threshold, getting closer to the 90 percent needed to make a bomb.

IAEA chief Rafael Grossi on Tuesday warned Iran's programme was "moving ahead very, very fast" and "growing in ambition and capacity".

But Tehran argues that the issues "are political in nature and should not be used as a pretext for abuse against Iran in the future".

"Now, the hours in Vienna are decisive and the Iranian side must be given assurances as soon as possible," an Iranian diplomat told the Iranian state news agency, IRNA.