The deal -- known as the JCPOA -- has been disintegrating since former US President Donald Trump dramatically withdrew from it in 2018 and re-imposed sanctions, prompting Iran to retaliate by exceeding the limits on nuclear activity laid down in the deal.

Diplomats from the remaining parties to the JCPOA -- Iran, China, Britain, France, Germany and Russia -- met this week in a luxury hotel in Vienna, with indirect American participation from a delegation based nearby.

The European Union is coordinating the talks and the chair of the meeting, EU diplomat Enrique Mora, said that the talks had been "constructive and results oriented".

Diplomats were "briefed on the work of the two expert groups on sanctions lifting and nuclear implementation measures", it said, referring to groups exploring actions that could be taken by Iran and the US to facilitate a return to the deal.

Moscow's ambassador to the UN in Vienna Mikhail Ulyanov also said that on Friday the remaining JCPOA participants "noted with satisfaction the initial progress made" and wanted to "maintain the positive momentum".

US President Joe Biden has said he is willing to return to the deal but Iran has refused direct negotiations with Washington as long as its sanctions against Tehran remain in place.

EU negotiators in Vienna have therefore been acting as intermediaries between parties to the main talks and American diplomats, who are based in a hotel close to the main talks.

Iranian Foreign Minister Mohammad Javad Zarif reiterated on Friday Tehran's position that the "US -- which caused this crisis -- should return to full compliance first," adding that "Iran will reciprocate following rapid verification".

The head of Iran's delegation to the talks Abbas Araghchi stressed the need for "political will and seriousness from other parties".

"Otherwise, there will be no reason to continue negotiations," he said, according to a statement from the Iranian foreign ministry.

The statement added that delegates from the remaining JCPOA countries would meet again in person on Wednesday.

The JCPOA put strict curbs on Iran's nuclear programme in order to reassure the international community Tehran would not be able to develop a nuclear weapon.

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