Talks restarted on Thursday to try to revive the 2015 deal between Iran and other nations, which the United States withdrew from under Donald Trump in 2018.

US President Joe Biden has said he is ready to return to the agreement but US officials have accused Iran of backsliding on progress made at talks earlier this year and playing for time.

Iranian officials maintain they are serious about committing to the talks.

"The two parties are at the point of agreeing on the matters which should be on the agenda," Tehran's chief negotiator Ali Bagheri told the official IRNA news agency.

"It's a positive and important evolution since, at the start, they weren't even in agreement on the issues to negotiate."

His comments came as the G7 said Sunday that time was running out for Iran to agree on a deal.

Foreign Secretary Liz Truss, from G7 host Britain, said the Vienna talks were the Islamic republic's "last chance to come to the negotiating table with a serious resolution".

Her comments were the first time a signatory to the original deal has given an ultimatum for the talks.

The 2015 agreement aimed to prevent Iran from developing an atomic bomb, a goal Tehran has always denied. The deal ensured sanctions relief in return for tight curbs on Iran's nuclear programme, which was put under extensive UN monitoring.

According to Bagheri, the new administration in power in Iran since June, led by ultraconservative President Ebrahim Raisi, has raised additional points to those negotiated by its reformist predecessors.

Iran wants a lifting of all US sanctions imposed after Trump's withdrawal but Biden's administration has said it will only negotiate measures taken by Trump over the nuclear programme, not steps imposed over other concerns such as human rights.

Tehran has presented two texts at the talks, one on the sanctions and the other on its nuclear activity.

"With the other parties we jointly recap (the points to discuss) in order that the differences can be resolved and the agenda established in a definitive framework," Bagheri said.