The European Union diplomat in charge of nuclear talks with Iran warned on Monday of "difficult" work ahead as the talks resumed in Vienna, AFP reported.
The envoy, Enrique Mora, said all sides were showing "a clear will to work toward the successful end of this negotiation".
"It's a very good sign... We will be working very seriously in the days and weeks ahead... It's going to be very difficult," Mora told reporters after the eighth round of talks hosted in Vienna began.
Ahead of the resumption of talks, Iran's Foreign Minister Hossein Amir-Abdollahian said the agenda should be "the issue of guarantees and verification" on the lifting of US sanctions.
"The most important thing for us is to reach a point where we can verify that Iranian oil will be sold easily and without any limits, that the money for this oil will be transferred in foreign currency to Iranian bank accounts, and that we will be able to benefit from all the revenues," he said, according to AFP.
Iran has gradually scaled back its compliance with the 2015 nuclear deal it signed with world powers in response to former US President Donald Trump’s withdrawal from the agreement in May of 2018.
The negotiations to salvage the agreement restarted in late November, after a five-month hiatus following the election of Iranian President Ebrahim Raisi.
The talks involve delegations from Iran and the other countries that remain party to the deal -- Britain, China, France, Germany and Russia.
Washington is participating indirectly, with diplomats shuttling back and forth between the Iranian and the US sides.
The Islamic Republic has repeatedly demanded that the US lift sanctions imposed on Iran and also reassure Iran it will not abandon the deal again as a precondition for its returning to compliance with the deal.
US officials have said that while they prefer the diplomatic route to reach an agreement with Iran, there are other options on the table should that fail.