Iran and six world powers will resume talks in Geneva on Thursday about how to implement a landmark nuclear agreement, a week after Tehran broke off the discussions in anger at an expanding U.S. sanctions blacklist.
Under the November 24 interim accord, Iran will curb its disputed nuclear program in exchange for a limited easing of sanctions that are damaging its oil-dependent economy.
The technical talks - expected to involve nuclear as well as sanctions experts - are meant to translate the political deal into a detailed plan on how to put it into practice.
Diplomats said the task was complicated but that progress had been made during the December 9-12 meeting in Vienna, even though differences remained. They said there was a real political will on both sides to carry out the agreement.
"It's in the interests of the Iranians to go quickly because there won't be an easing of sanctions until the agreement is implemented," a senior Western diplomat said.
In a sign of this, deputy Iranian chief negotiator Abbas Araqchi said the expert talks were set for an initial two days but may continue into Saturday and Sunday if required, Iran's Fars news agency said.
A spokeswoman for European Union foreign policy chief Catherine Ashton, who coordinates contacts with Iran on behalf of the powers, confirmed the resumption of the discussions.
Last Thursday, Iranian negotiators interrupted the talks in Vienna in protest against the U.S. blacklisting of an additional 19 Iranian companies and individuals under existing sanctions, saying the move was against the spirit of the deal.
U.S. officials said the move did not violate the Geneva agreement and that they gave Iran advance warning.
The development has highlighted the sensitivities involved in implementing the agreement. Some U.S. lawmakers are pushing for further sanctions against Iran, a move which hardliners in Iran see as proof the United States cannot be trusted.
The six powers - the United States, Russia, China, Britain, France and Germany - are seeking to scale back Iran's atomic program to prevent it from developing nuclear weapons. Iran denies any such intention, saying it needs nuclear power in order to generate electricity.
The Geneva 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 Rohani as president in June. He had promised to reduce Tehran's isolation and win sanctions easing.
Separately, diplomats in Vienna said the UN nuclear agency could face costs of roughly $6.9 million to verify that Iran lives up to the deal.
The 35-nation governing board of the UN International Atomic Energy Agency is expected to hold an extraordinary meeting next month to discuss the IAEA's expanded role.
The extra cost is unlikely to create any major difficulty in view of the political importance of resolving the dispute.
However, diplomats accredited to the UN agency said it could be sensitive as IAEA Director General Yukiya Amano likely needs both to seek member state help to pay for more inspections in Iran and find some of the money internally.
The IAEA's budget for 2014 is around $473 million.
The IAEA - tasked with preventing the spread of nuclear weapons - regularly inspects Iranian nuclear sites to make sure there is no diversion of atomic material for military purposes.
But it will step up the frequency of its visits to the uranium enrichment sites of Natanz and Fordow under the Geneva agreement and also carry out other additional tasks in inspecting nuclear-linked facilities.
The agency has two to four staff in Iran virtually every day of the year, with some 20 dedicated to inspector activity there, but that number is now likely to rise.
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