Western parties to the 2015 nuclear agreement have accepted to give priority to the sanctions issue in the Vienna talks, Iranian top negotiator said on Tuesday.

"Addressing the situation of the iniquitous and illegal sanctions must be the first priority issue in the agenda of the Joint Commission, and this was agreed at this meeting," Ali Bagheri Kani said in remarks to Iran's state TV, regarding the first day of the renewed negotiations in Austria's capital.

Therefore, he added, a workgroup focused on the sanctions would meet on Tuesday morning.

Concerning the prospects of the negotiations, Bagheri Kani said "many" of Iran's counterparts welcomed the Iranian viewpoints on the need for guarantees that the United States will not reimpose again sanctions on Iran that are supposed to be lifted under the 2015 agreement, and that Washington will not quit the deal as it did in 2018 under former President Donald Trump.

The Iranian diplomat described Monday's meetings as "constructive and useful," and praised the Chinese and Russian representatives' support for Iran's stances.

European representatives, he added, acknowledged in their remarks that the main cause of the present situation is the U.S. withdrawal from the agreement, and therefore Washington should be the first to take action in order to revive the deal.

Concerning the six previous rounds of talks, Bagheri Kani said all the parties in Monday's meeting admitted the principle that "nothing is agreed until everything is agreed," and therefore any of the preliminary outcomes drafted between April and June could be reviewed.

Iran and the remaining parties to the 2015 nuclear deal, with indirect involvement of the United States, resumed the talks on Monday, after nearly six months of hiatus.