Iran says it will take the second step in reducing its commitments under the 2015 nuclear deal more “firmly” if a European payment system designed to bypass sanctions proves to be "superficial".

Foreign Ministry spokesman Abbas Mousavi said Friday's meeting in Vienna between the remaining signatories of the nuclear deal is the "last chance" to save the accord after the US exit last year.

Iran demands "practical" steps be taken on operationalizing the planned European trade mechanism, or Instex, when senior officials meet in the Austrian capital with the aim of saving the nuclear accord.

"I believe this meeting could be the last opportunity for the remaining signatories of the JCPOA to convene and seek how they can implement their commitments toward Iran," said Mousavi.

European signatories to the Joint Comprehensive Plan of Action (JCPOA) are facing a two-month ultimatum to help Iran navigate US sanctions or see Tehran take the second step of reducing its commitments on July 7.

In early May, Tehran suspended limits on its production of enriched uranium and heavy water, moves that did not technically violate the deal but signaled that its patience was wearing thin.

Last week, the spokesman for the Atomic Energy Organization of Iran said Iran will surpass the uranium stockpile limit set under the nuclear deal from June 27.

Mousavi said that despite supporting Iran's stance in several statements, the remaining signatories - Britain, China, France, Germany and Russia - had failed to take any action.

"We hope that the remaining members of the JCPOA, in spite of the slogans they have chanted and the good statements they have issued, will take a practical and effective step this time to persuade Iran to fulfill its obligations."

France, Germany and Britain have been tinkering with Instex for months, but have failed to operationalize it, leaving Iran wondering whether they are serious about the idea.

Mousavi said after the US withdrawal in May 2018, Iran resorted to its "strategic patience" as it decided to remain in the deal and give diplomacy a chance.

"However, they dragged their feet, and the mechanism has taken until now to be operational, which we blame on possible US opposition and the lack of will on the part of the Europeans and their inaction."

After the recent ultimatum set by Iran, the Europeans have taken certain steps, Mousavi said, adding "we do not deny these efforts, but we do not think they are enough."

"We need to see how Instex will work and how it will be operational, and time is also important for Iran," he added.

The amount of financial transfers made through Instex is also important for Iran, Mousavi said, stating that Iran used to sell about 2.8 million barrels per day of oil and receive the revenues before the US pullout.

"If the Europeans take a step and operationalize Instex, it should meet our needs. But if we realize that this is a commonplace and superficial mechanism, Iran will definitely not accept it and will firmly take the second step at the set time."

Britain, Germany and France unveiled the direct non-dollar payment mechanism in late January, but it has not resulted in any trade deals.

Mousavi made the comments as a JCPOA Joint Commission meeting, headed by Araqchi and EU deputy foreign policy chief Helga Schmidt, was expected to gather representatives from all of the deal's current signatories on Friday.

Speaking in Vienna, Araqchi said Iran has ran out of "strategic patience". Iran will not tolerate remaining unilaterally committed to the nuclear deal, he said, adding Tehran seeks "tangible" action from the other signatories.

Media reports also said Iranian Foreign Minster Mohammad Javad Zarif has written a letter to EU Foreign Policy Chief Federica Mogherini, warning that Tehran reserves the right to suspend its obligations and that only mutual commitment to the deal could save the accord.

Mogherini said on Tuesday Instex was “ready for being operational” and expressed hope that the effort could persuade Iran to continue its compliance with the JCPOA.