EU diplomat Enrique Mora hailed "progress made over the last two weeks" but said "ongoing challenges" remained to put the accord -- which curbs Iran's nuclear programme in exchange for sanctions relief -- back on track.

But for Iranian President Hassan Rohani, the "negotiations have achieved 60-70 percent progress," according to IRNA news agency.

"If the Americans act honestly, we will reach a conclusion in little time," Rohani was quoted as saying.

The deal was thrown into question when the US withdrew in 2018 and sanctioned Iran, which in turn started ramping up its nuclear activities.

US President Joe Biden is seeking to revive the agreement.

Diplomats from Britain, China, France, Germany, Iran and Russia have been meeting regularly since early this month in a luxury Vienna hotel, while US diplomats are participating indirectly in the talks from a nearby hotel.

"Much more hard work needed," Mora wrote on Twitter after a fresh round of talks.

He added a third expert group had been created to address "sequencing issues" after two groups were set up to look into nuclear issues and sanctions lifting.

Iran insists it can reverse its nuclear activities -- including producing uranium enriched to 60 percent purity, far above the deal's threshold -- but only once sanctions imposed by former US president Donald Trump are removed.

Talks are set to continue next week, according to an EU statement, with participants noting a new sense of urgency.

Mikhail Ulyanov, Russia's ambassador to Vienna, wrote on Twitter that the parties "decided to take a break to allow the delegations to do homework and consult with the capitals".

Ulyanov on Monday had said negotiations had entered "the drafting stage" though "practical solutions" were "still far away".