While Iran announced a major concession on Saturday, a major obstacle to the Vienna nuclear talks has emerged.

Russia is now threatening to block the planned return to the 2015 nuclear agreement.

Foreign Minister Sergei Lavrov on Saturday demanded "guarantees" that Russia could trade freely with Iran despite sanctions imposed over its incursion into Ukraine.

This puts a considerable strain on an “Iran deal”, for which a foreign ministers' meeting in Vienna had been expected these days.

Stephen Lowenstein

Political correspondent based in Vienna.

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Positive signals had previously come from Tehran that Iran was ready to meet the demands of the International Atomic Energy Agency (IAEA).

"We have decided that we will provide the IAEA with the necessary documents by June of this year," said the head of Iran's nuclear program, Mohammad Eslami, on Saturday at a joint press conference with IAEA Director General Rafael Grossi in Tehran.

This step is intended to enable constructive and professional cooperation between Iran and the IAEA in the future.

Grossi pointed out that the IAEA is obliged to investigate open questions, regardless of the sources from which the information leading to the questions came.

The open questions concern nuclear residues that IAEA inspectors found at two locations in Iran that were not declared as sites of the nuclear program.

According to media reports, the references to these places had come from Western secret services.

What seems clear, however, is that these are traces of past activity, not signs of a current clandestine offshoot of the nuclear program.

Iran had therefore rejected these questions as "politicized".

At the Vienna negotiations, Iran had set the condition that a line had to be drawn under the past.

The Western negotiators could not concede that because they cannot make any commitments on behalf of the IAEA.

Grossi had now traveled to Tehran to seek clarification on his part.

The joint appearance indicates that both sides agree with the procedure that has now been announced.

Material for two bombs within a month

But now the intervention came from Moscow.

Lavrov said Saturday the deal with Iran is nearing completion.

However, "problems have recently arisen from the point of view of Russia's interests".

Lavrov referred to the "avalanche of aggressive sanctions unleashed by the West over the Ukraine conflict."

Moscow now needs "written guarantees" from the United States that sanctions will not affect Russia's rights under the nuclear deal.

It is about "unrestricted trade, economic and investment cooperation as well as military-technical cooperation with Iran".