Iran has quit nuclear talks with world powers, accusing Washington on Friday of going against the spirit of a landmark agreement reached last month by expanding its sanctions blacklist.
The future of Iran's Arak heavy water IR-40 reactor is one of the key points in a landmark nuclear deal Tehran recently signed with world powers in Geneva
US Secretary of State John Kerry and a spokesman for EU foreign policy chief Catherine Ashton, who represents the major powers in the talks, both played down the suspension and said talks were expected to resume soon.
But Iran's chief nuclear negotiator Abbas Araqchi said the US move went against the spirit of the deal struck in Geneva under which the powers undertook to impose no further sanctions for six months in exchange for Iran curbing its controversial nuclear activities.
Tehran was now weighing the "appropriate response," Araqchi told the Fars news agency as his team headed back to Tehran from Vienna.
Iran's ambassador to France, Ali Ahani, warned at a press conference in Monaco that such moves could serve to boost opposition within his country to a deal with the West.
"We are determined to implement our commitments. We must be sure that the other side is serious, and that we can show our people that we can trust them and that Western countries are reliable partners," he said.
A senior official of the elite Revolutionary Guards, Brigadier General Yadollah Javani, told Fars: "Government officials should ... take practical measures proportionate to the act of the other party."
Under the deal, Iran agreed to freeze parts of its suspect nuclear programme for six months in return for some $7 billion in relief from Western sanctions as it negotiates a final, comprehensive accord to allay suspicions it seeks a weapons capability.
Iranian negotiators quit the implementation talks late on their fourth day Thursday after Washington blacklisted a dozen companies and individuals for evading US sanctions.
Foreign ministry spokeswoman Marzieh Afkham was quoted by state news agency IRNA as saying "the consequences of this unstudied move (are) completely on the American government."
The Mehr news agency quoted informed sources as saying that the "new American sanctions" were the reason for the interruption.
"The negotiations were halted by the Iranian delegation because of new American sanctions. The Iranian negotiating team has halted the talks at this stage," it said.
Kerry says talks making progress
Kerry, however, made no mention of a breakdown in the talks in remarks to reporters as he concluded a trip to Israel.
"We’re making progress, but I think we’re at a point in those talks where folks feel a need to consult, take a moment," he said.
"There is every expectation that talks are going to continue in the next few days."
A spokesman for the EU foreign policy chief also said both sides needed to return home for consultations and that Iran's move was not unilateral.
"After four days of lengthy and detailed talks, reflecting the complexity of the technical issues discussed, it became clear that further work is needed," Ashton's spokesman Michael Mann said.
"There will now be consultations in capitals, in the expectation that technical talks will continue soon," he added.
Under Iran's deal with the P5+1 -- Britain, China, France, Russia and the United States plus Germany -- Washington agreed to refrain from imposing new sanctions on Iran.
But senior administration officials argued that Thursday's blacklistings were carried out within the framework of the existing sanctions regime, which had forced Tehran to the negotiating table, and did not constitute new measures.
The blacklisting of a dozen additional foreign firms and individuals for evading US sanctions was widely seen as a way to head off moves in Congress to impose additional sanctions that would be in clear breach of the Geneva agreement.
Administration officials insisted the timing was entirely coincidental.
But just hours afterwards, Senate banking committee chairman Tim Johnson and the committee's top Republican Michael Crapo agreed with the White House that Washington should not introduce new sanctions, warning they could "rupture" international unity against Tehran's nuclear programme.
The comments virtually assured that no new sanctions legislation would pass Congress before the year-end break, although lawmakers could introduce a new sanctions bill within the next week.