The talks between the remaining partners to the deal -- Iran, China, Russia, Germany, France and the UK -- will restart at around two pm local time (1300 GMT), the European diplomat told AFP on Sunday.

The US will also send a delegation headed by Washington's Special Envoy for Iran, Rob Malley, which will participate in the talks indirectly.

Russia's ambassador to the UN in Vienna Mikhail Ulyanov had said this week that "informal meetings" between the participants were expected ahead of the formal talks beginning in the Austrian capital's Palais Coburg hotel.

Ulyanov noted that Monday would mark more than five months since the talks were suspended, "a very protracted pause".

"The talks can't last forever. There is the obvious need to speed up the process," Ulyanov tweeted on Saturday.

On Saturday Iranian media reported that the country's chief nuclear negotiator Ali Bagheri had arrived in Vienna, days after visiting Kuwait and the United Arab Emirates.

The 2015 deal known as the Joint Comprehensive Plan of Action, or JCPOA, was designed to prevent Iran from acquiring a nuclear arsenal by imposing strict limits on its nuclear programme in return for the lifting of some sanctions.

But it unravelled in 2018 when then president Donald Trump unilaterally withdrew the US from the accord.

A year later Iran began retaliating by rolling back its nuclear-related commitments, for example breaching the limits laid down in the accord on its stockpile of enriched uranium.

Trump's successor Joe Biden has said he wants the US to return to the accord but Washington has accused Iran of dragging its feet and making "radical" demands.