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

The deal was thrown into question when the US withdrew from it 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 was created to address "sequencing issues" after two groups had already been set up earlier 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 to continue next week, according to an EU statement.

Russian ambassador to Vienna Mikhail Ulyanov 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 that the negotiations had entered "the drafting stage" though "practical solutions" were "still far away".