The diplomats are scheduled to meet from 2:00 pm local time (1300 GMT) in a luxury hotel in downtown Vienna, according to the EU, which is chairing the talks.

The talks -- aimed at bringing the US back into the agreement and Iran to roll back its nuclear activities -- started in April this year, but then stopped for several months as the Islamic republic elected a new hardline government.

The talks finally resumed in late November.

"We have made good progress this week. We will convene a joint commission today and will continue talks after a break of a few days," Tehran's chief negotiator Ali Bagheri said on Twitter, referring to the meeting's formal name.

EU and US diplomats have been more guarded in their comments so far.

Diplomats from Britain, France and Germany -- all party to the deal -- said on Monday that "real negotiations" had yet to start in the current round, while "precious time" was being lost "dealing with new Iranian positions inconsistent" with the agreement.

Washington -- which under then president Donald Trump pulled out of the deal in 2018 and reinstated sanctions on Iran -- is indirectly participating in the talks.

The negotiations also include China and Russia, both parties to the agreement, which was meant to offer Iran sanctions relief in exchange for curbs on its nuclear programme.

Following the US pullout, Iran has taken successive steps to increase its nuclear activities.

Earlier this year, Tehran also began restricting some inspection activities by the UN nuclear watchdog, the International Atomic Energy Agency (IAEA).

Tehran and the Vienna-based IAEA announced on Wednesday that they had reached agreement on replacing the cameras at a facility which makes centrifuges.

But the IAEA will not be able to examine the camera images until after sanctions are lifted, an Iranian official said on Thursday.

IAEA director general Rafael Grossi is expected to hold a press conference at noon local time (1100 GMT).