VIENNA/MOSCOW A breakthrough agreement to end a standoff over Iran's nuclear programme appeared to face its first major difficulty on Friday with Russia warning that a U.S. sanctions move could "seriously" complicate its implementation.
Russia, which along with the United States is among the six world powers which negotiated the November 24 interim accord with Tehran, echoed Iran's criticism by saying Washington's sanctions decision violated the spirit of the deal.
Moscow's statement came after diplomats said Iran had interrupted technical talks with the six nations in Vienna over how to implement the agreement, under which Tehran is to cap its nuclear programme in return for limited sanctions easing.
The developments highlighted potential obstacles negotiators face in pressing ahead with efforts to resolve a decade-old dispute between the Islamic Republic and the West that has stirred fears of a new Middle East war.
Several Western diplomats insisted the inconclusive outcome of the December 9-12 expert-level discussions in Vienna should not be seen as a sign that the political deal hammered out nearly three weeks ago was in serious trouble.
But Russia made its concerns clear a day after the United States blacklisted additional companies and people under existing sanctions intended to prevent Iran from obtaining the capability to make nuclear weapons. Iran denies any such aims.
"The U.S. administration's decision goes against the spirit of this document," said Foreign Ministry spokeswoman Maria Zakharova, referring to the Geneva agreement between Iran and the United States, Russia, China, France, Britain and Germany.
"Widening American 'blacklists' could seriously complicate the fulfilment of the Geneva agreement, which proposes easing sanctions pressure."
Russia built Iran's first nuclear power plant and has much better ties with Tehran than Western states. It supported four rounds of U.N. Security Council sanctions aimed at reining in Tehran's nuclear programme but has criticised the United States and Europe for imposing additional sanctions.
U.S. officials said the blacklisting move showed the Geneva deal "does not, and will not, interfere with our continued efforts to expose and disrupt those supporting Iran's nuclear programme or seeking to evade our sanctions".
The new measure, the first such enforcement action since Geneva, targeted entities that are suspected of involvement in the proliferation of materials for weapons of mass destruction and trying to evade the current sanctions.
Some U.S. lawmakers want further sanctions on the Islamic state. But the administration of U.S. President Barack Obama has campaigned to hold off on new measures for now to create space for the diplomatic push to settle the nuclear row.
Iranian Deputy Foreign Minister Abbas Araqchi told the semi-official Fars news agency in reaction to the U.S. decision: "We are evaluating the situation and Iran will react accordingly to the new sanctions imposed on 19 companies and individuals. It is against the spirit of the Geneva deal."
Foreign Ministry spokeswoman Marzieh Afkham said the U.S. government's "unconstructive, repetitive and useless measure" was in contrast with efforts to dissuade lawmakers from adopting new sanctions.
Experts from Iran, the six powers and the European Union laboured during four days of discussions at the headquarters of the U.N. nuclear watchdog in the Austrian capital to agree on how to carry out the Geneva accord in practice.
U.S. Secretary of State John Kerry said he expected talks to resume in the coming days. "We have been hard at it in Vienna ... we are making progress but I think that they're at a point in those talks where folks feel a need to consult and take a moment," he said during a visit to Israel.
"There is every expectation that the talks are going to continue in the next few days and that we will proceed to the full implementation of that plan."
The experts will now consult with governments. A spokesman for EU foreign policy chief Catherine Ashton, who coordinates the talks, also said talks were expected to resume soon.
"After four days of lengthy and detailed talks, reflecting the complexity of the technical issues discussed, it became clear that further work is needed," Michael Mann said.
Under the Geneva agreement, Iran will restrain its atomic activities in return for some easing of the international sanctions that have battered the major oil producer's economy.
One diplomat said the Iranian delegation suddenly announced late on Thursday evening - hours after Washington made its blacklisting decision public - that it was returning to Tehran.
The Iranians said "they had received instructions from Tehran to stop the discussions and fly back to Tehran", the diplomat said. "It was quite unexpected."
An EU diplomat said he did not believe the decision was linked to the issues under discussion in Vienna, but rather "their reaction to moves in the U.S. on sanctions".
The hope was that it was a temporary problem. "The Iranians have been committed to making this work. We are not panicking," the diplomat said.
Iranian officials were not available for comment.
Diplomats earlier said it was very hard to translate the Geneva deal into an action plan, but that there was a political will to iron out any differences. They said implementation may start in January after technical matters have been settled.
The deal was designed to halt Iran's nuclear advances for six months to buy time for negotiations on a final settlement.
Scope for diplomacy widened after Iran elected the pragmatic Hassan Rouhani as president in June. He had promised to reduce Tehran's isolation and win sanctions easing.
(Additional reporting by Parisa Hafezi in Ankara and Adrian Croft in Brussels; editing by Andrew Roche)
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