By Fredrik Dahl and Adrian Croft
VIENNA/BRUSSELS (Reuters) - Iran's anger over a new U.S. sanctions measure may have been behind its move to interrupt talks with world powers over how to implement last month's breakthrough nuclear agreement, two diplomats said on Friday.
Several envoys stressed, however, that 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.
The November 24 interim accord, reached after marathon talks in Geneva, is seen as a step towards resolving a decade-old standoff over suspicions that Iran is covertly seeking the capability to make nuclear weapons, a charge Tehran denies.
Experts from Iran, the United States, France, Germany, Britain, China, Russia and the European Union labored during this week's talks at the headquarters of the U.N. nuclear watchdog to agree on how to carry it out in practice.
But in a sign of the technical difficulties involved, they will now consult with their capitals before meeting again.
A spokesman for EU foreign policy chief Catherine Ashton, who coordinates the six powers' negotiations with Iran, said the discussions were expected to resume in the near future.
"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.
"There will now be consultations in capitals, in the expectation that technical talks will continue soon."
Iran said on Friday a new U.S. measure targeting companies and individuals for supporting its nuclear program violated the spirit of the Geneva deal.
Under that deal, Iran will curb its atomic activities in return for some easing of the international sanctions that have battered the major oil producer's economy.
The United States on Thursday black-listed additional companies and people under sanctions aimed at preventing Iran from obtaining the capability to make nuclear weapons, U.S. officials said. Iran says its atomic work is purely peaceful.
Treasury and State Department officials said the move showed the Geneva deal "does not, and will not, interfere with our continued efforts to expose and disrupt those supporting Iran's nuclear program or seeking to evade our sanctions."
Iranian Deputy Foreign Minister Abbas Araqchi told the semi-official Fars news agency on Friday: "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."
One diplomat said the Iranian delegation suddenly announced late on Thursday evening - hours after Washington made its 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 the Austrian capital.
"My understanding was that this was not to do with a specific problem in what they were talking about but actually their reaction to moves in the U.S. on sanctions," the diplomat said, adding that the hope was that it was a temporary problem.
"The Iranians have been committed to making this work," the diplomat said. "We are not panicking."
Iranian officials were not immediately available for comment.
Diplomats earlier said it was very hard to translate the Geneva agreement into a plan for action, 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 of a dispute that has stirred fears of a new Middle East war.
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)