Western European participants have warned that time is running out for a successful conclusion to the talks.
Diplomats from Iran and world powers have reconvened in Vienna to seek a deal reviving Tehran’s 2015 nuclear accord.
Pressure has been mounting for a swift result from the final stage of the talks, as Iran steps up its nuclear programme.
Tehran has insisted that its actions are peaceful, but Western diplomats argue that the deal could soon become an “empty shell”.
The United Kingdom, France, Germany, Russia, and China were parties to the agreement as well as Iran’s chief negotiator, Ali Bagheri Kani.
The United States has participated indirectly in the talks because it withdrew from the accord in 2018 under former President Donald Trump. His predecessor 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 programme.
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.
Under Trump, the US reimposed heavy sanctions on Iran. Tehran has responded by increasing the purity and amounts of uranium it enriches and stockpiles, in breach of the nuclear deal.