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 -- have been meeting in a luxury hotel in Vienna, with indirect American participation from a delegation based nearby.

Moscow's ambassador to the UN in Vienna Mikhail Ulyanov said Friday the remaining JCPOA participants "took stock of the work done by experts over the last three days and noted with satisfaction the initial progress made".

He added in a tweet that diplomats "will reconvene next week in order to maintain the positive momentum".

Iran and the US had also described the first day of talks on Tuesday as "constructive".

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.

In the meantime, two expert-level groups -- on sanctions lifting and nuclear issues -- have been meeting over the course of the week to explore the actions that could be taken by Iran and the US to facilitate a return to the deal.

The 2015 deal, known as 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.