The Director-General of the International Atomic Energy Agency, Rafael Grossi, will hold talks with Iranian officials in Tehran on Saturday, amid an atmosphere of cautious optimism about the chances of reaching an understanding on the nuclear deal.

"It is expected to review general issues between us and the agency regarding how to pursue various issues in the future... God willing, there will be an understanding," said Behrouz Kamalvandi, spokesman for Iran's Atomic Energy Organization, in a statement to state television on Saturday.

The spokesman added that Grossi will also meet with Iranian Foreign Minister Hossein Amir Abdollahian, before returning to Vienna this afternoon.

Grossi arrived in the Iranian capital on Friday evening, on a visit that comes in parallel with the efforts being made in Vienna to save the 2015 nuclear agreement.

The Director-General of the International Atomic Energy Agency, in a tweet on Twitter prior to his arrival, said that "the timing is sensitive, but a positive result can be achieved for all."

And he had confirmed last Wednesday that the agency will never give up its efforts to push Tehran to provide explanations about the presence of nuclear materials in undeclared sites on its soil.

For its part, Tehran asserts that achieving an understanding in the Vienna talks will not be achieved unless this file is closed, which it places in the context of "political allegations" against it.

On Friday, European Union foreign policy chief Josep Borrell said he hoped to "achieve results at the weekend" in order to revive the 2015 agreement, but at the same time stressed that there were still points of contention.

For her part, the head of the British negotiating delegation, Stephanie Al-Qaq, said that the European diplomats in Vienna will return to their capitals soon for consultations.

"We are close" to an agreement, she said, explaining that the negotiators of the three European signatories, Germany, France and Britain, "will leave Vienna soon to inform the ministers, and are ready to return soon."

The three European countries, along with Russia and China, have been participating for months in the Vienna talks, which are taking place with the indirect participation of the United States.

On Thursday, the three countries said a potential agreement was imminent, and called for outstanding issues to be resolved quickly.