Among them was Iran's chief negotiator, Ali Bagheri Kani, who returned to Austria after consultations with his government in Tehran over the weekend.
The other parties to the agreement are the United Kingdom, France, Germany, Russia and China.
The United States has participated indirectly in the talks because it withdrew from the accord in 2018 under then-president Donald Trump.
US President Joe Biden has signalled that he wants to rejoin the deal.
The accord, formally known as the Joint Comprehensive Plan of Action, granted Iran sanctions relief in exchange for curbs on its nuclear program.
Under Trump, the US reimposed heavy sanctions on Iran.
Iran has responded by increasing the purity and amounts of uranium it enriches and stockpiles, in breach of the accord.
Officials say that the talks are reaching their final stage although it is unclear how long that might take.
The negotiations have dragged on for months, punctuated by a long gap last year caused by the arrival of a hard-line new government in Iran.
The three western European participants have been warning for a while that time is running out for a successful conclusion of the talks.
They have argued that the accord will soon become an "empty shell" in light of Iran stepping up its nuclear program.
Iran insists that its program is peaceful.
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