The latest round of talks began last week and were paused on December 3 with Western participants accusing Iran of going back on progress made earlier this year.

The heads of delegations from the parties to the 2015 deal -- Britain, China, France, Germany, Iran and Russia -- were present at Friday's talks, which began at the Palais Coburg luxury hotel at around 12 pm (1100 GMT) and lasted a little more than an hour.

An American delegation plans to take part in the talks indirectly in the coming days.

"Delegations took a stack of the different consultations among capitals and they have come with a renewed sense of purpose to work hard," Enrique Mora, the EU official chairing the talks, told the press after Thursday's meeting.

Bilateral meetings as well as expert working groups are expected to continue this week.

Mora admitted that the negotiations were "a very difficult endeavour", adding: "There are still different positions that we have to marry".

Russia's ambassador to the UN in Vienna Mikhail Ulyanov told the TASS agency that Thursday's talks had "removed a number of misunderstandings that had created some tension," but did not elaborate.

The current round of talks is the seventh since they started in April.

In June, Iran suspended them following the election of ultraconservative President Hassan Rouhani and they were only restarted on November 29.

US envoy Rob Malley "will plan to join the talks over the weekend," said US State Department spokesman Ned Price on Wednesday.

"We should know in pretty short order if the Iranians are going... to negotiate in good faith," Price told reporters, warning that "the runway is getting very, very short for negotiations."

- 'Media campaigns' -

For their part Iranian officials have insisted they are "serious about the talks".

"The fact that the two sides are continuing to talk indicates that they want to narrow the gaps," said Iran's chief negotiator Ali Bagheri.

The EU's top foreign policy official Josep Borrell spoke to Iran's Foreign Minister Hossein Amir-Abdollahian on Wednesday evening.

According to the Iranian foreign ministry website, Borrell asked Tehran to "respond to worries concerning its current nuclear programme", which has intensified in recent months.

Iranian officials have in recent days condemned the "negative" reaction of Western states after last week's meetings, saying that "such media campaigns are not constructive".

The 2015 deal has been disintegrating ever since then US President Donald Trump pulled out of the deal, which ensured sanctions relief for Iran in return for tight curbs on its nuclear programme, which was put under extensive UN monitoring.

Trump went on to re-introduce sanctions, prompting Tehran to start disregarding the deal's limits on its nuclear activities in 2019.

Trump's successor Joe Biden has said he wants the US to return to the deal but the talks have stumbled on which sanctions Washington is prepared to lift as well as guarantees demanded by Iran to protect against the prospect of a future US withdrawal.