The negotiations -- attended by Britain, China, France, Germany, Iran, Russia and indirectly the United States -- were halted at the end of last month.

The resumption of talks comes after parties in recent weeks cited progress in seeking to revive the 2015 accord that was supposed to prevent Iran from acquiring an atomic bomb, a goal it has always denied pursuing.

"A deal that addresses all sides' core concerns is in sight, but if it is not reached in the coming weeks, Iran's ongoing nuclear advances will make it impossible for us to return to the JCPOA," a US State Department spokesperson said on Monday, referring to the 2015 framework agreement.

Parties have been negotiating in Vienna, with indirect US participation, since last year.

A source close to the discussions told AFP that the delegations had arrived in the Austrian capital and that the discussions were set to resume in the afternoon at the upmarket Coburg Palace hotel.

The United States, under former president Donald Trump, unilaterally withdrew from the pact in 2018 and reimposed tough economic sanctions on Iran, prompting the Islamic republic to begin pulling back from its commitments under the deal and step up its nuclear activities.

Iranian foreign ministry spokesman Saeed Khatibzadeh said on Monday that answers that "the United States brings... to Vienna will determine when we can reach an agreement."

"We have made significant progress in various areas of the Vienna negotiations" including on guarantees sought by Iran that the United States would not breach the deal once again, Khatibzadeh told reporters.

- 'Decisive moment' -

Experts say the Iranians have deviated so much from the restrictions of the 2015 deal that they are only weeks away from having enough fissile material to make an atomic weapon.

Washington has sought direct negotiations in this home stretch, but said talks remain indirect at Iran's request.

German Chancellor Olaf Scholz called it "the decisive moment" in an interview with the Washington Post published on Monday.

"We gave them a clear message that now this is the time for decisions and for progress, and not for prolonging the process," he said. "We hope that they will use the chance."

"We are five minutes away from the finish line," Russian negotiator Mikhail Ulyanov said in an interview to the Russian daily Kommersant.

"A draft of the final document has been crafted. There are several points there that need more work, but that document is already on the table," he continued.

On Friday, Washington made a gesture by announcing it was waiving sanctions on Iran's civil nuclear programme, a technical step necessary to return to the 2015 Joint Comprehensive Plan of Action, or JCPOA.

The waiver allows other countries and companies to participate in Iran's civil nuclear programme without triggering US sanctions, in the name of promoting safety and non-proliferation.

The move "should facilitate technical discussions necessary to support talks on JCPOA return in Vienna," negotiators of Britain, France and Germany said in a joint statement Saturday.

For Iran, though, the move fell short.

"Continuing maximum pressure against #Iran, current US administration has so far tried to meet the goals that Trump failed to achieve through bullying, by making unsupported promises.

"With this Washington's illusions, the path to negotiations will not be smooth" Ali Shamkhani, secretary of Iran's Supreme National Security Council, said in a tweet written in English on Tuesday.

burs-anb/ach/jm