Negotiators from Iran and five world powers have resumed talks on restoring Tehran's tattered 2015 nuclear deal.
Iran is insisting that the United States and its allies promise to allow it to export its crude oil.
It is the eighth round of talks in Vienna, which opened 10 days after they were adjourned for Iran's representative to go home for consultations.
The previous exchange was the first after a gap of more than five months, due to the arrival of Iran's new hard-line government, and it was marked by tensions over fresh demands from Tehran.
The EU diplomat who chaired the talks, Enrique Mora, said after the opening session: "If we work hard in the days and weeks ahead, we should have a positive result.
"It's going to be very hard – difficult political decisions have to be taken."
Tehran's landmark accord with world powers: Britain, France, Germany, the US, Russia and China; granted Iran sanctions relief in exchange for curbs on its nuclear programme.
But in 2018, then-President Donald Trump withdrew America from the deal and imposed sweeping sanctions on Iran, including against its oil sector – the lifeline of its economy.
Iran's crude exports plummeted, and international oil companies scrapped deals with Tehran, weakening its economy.
The other signatories struggled to keep alive the agreement, formally known as the Joint Comprehensive Plan of Action.
The United States is participating only indirectly in this year's talks to restore the deal, which President Joe Biden has signalled he wants to rejoin.
Speaking in Tehran ahead of the meeting, Iran's foreign minister, Hossein Amirabdollahian, said Iran wants the upcoming round of discussions talks to focus on its sanctions-hit oil industry.