By Tom Miles and Justyna Pawlak
GENEVA/BRUSSELS (Reuters) - Iran and six world powers resumed expert-level talks in Geneva on Thursday to work out how to put into practice a landmark deal obliging Tehran to curb its nuclear program in return for some relief from economic sanctions.
Discussions on the implementation details of last month's breakthrough accord were interrupted by Tehran diplomats last week, after a decision by the United States to blacklist 19 more Iranian companies and individuals.
But diplomats said much progress had been achieved in the four-day meeting on December 9-12 in Vienna and expressed hope they could wrap up the practical discussions at meetings in Geneva on Thursday and Friday.
That could mean the seven countries - the United States, Russia, China, France, Britain, Germany and Iran - would be ready to agree on a date when the accord goes fully into effect.
Specifically, they would decide when western governments ease sanctions and how much prior verification of any Iranian curbs of its most sensitive nuclear work would be needed ahead of time.
Diplomats have said their target time frame was the second half of January, possibly on the day of the next meeting of EU foreign ministers on January 20, who could approve easing of EU sanctions.
"We were at an advanced stage in Vienna," said a diplomat from one of the six world powers. "A lot of work has been done so we can go very fast."
The diplomat, who spoke on condition of anonymity, cautioned, however, that some western diplomats were concerned Iran could be "more difficult" in the technical discussions because of Washington's decision to expand sanctions this month.
"I am afraid the Iranians will be tougher now," he said.
The nuclear accord is designed to halt Iran's nuclear advances for a period of six months to buy time for negotiations on a final settlement of the decade-old standoff.
Iran rejects western suspicions that its atomic work is aimed at acquiring nuclear weapons and says it is for peaceful purposes only.
U.S. officials have said the new blacklistings should not complicate the practical talks and are part of U.S. efforts to continue exposing those supporting Iran's nuclear program or seeking to evade current sanctions.
Underscoring Iranian concerns, Deputy Foreign Minister Abbas Araqchi travelled to Brussels this week, in part, to discuss the U.S. decision with the EU's foreign policy chief, Catherine Ashton, who oversees contacts with Iran on behalf of the powers.
In the technical discussions, experts aim to resolve issues dealing with how exactly sanctions can be lifted and what specifically Iran must do to meet its obligations on suspending parts of its nuclear work.
Diplomats said some issues had already been resolved in Vienna last week, including some aspects of how the U.N. nuclear watchdog, the International Atomic Energy Agency, will verify what Iran has done before the deal can be put into effect.
Questions remain on how western governments will ensure banks understand what transactions are allowed under the softened sanctions regime, and how and when Iran will be allowed to access several billion dollars worth of oil revenues frozen in overseas accounts.
Under discussion are issues such as the technical details of how Iran will limit its enrichment of uranium to less than 20 percent, a level that constitutes a major technological leap en route to producing weapons fuel.
Iran has agreed to suspend enriching uranium to 20 percent under the November accord.
Scope for easing the dispute peacefully opened after the June election of a comparative moderate, Hassan Rouhani, as Iranian president. He won in a landslide by promising to lessen Tehran's international isolation and win relief from sanctions that have severely damaged the oil producer's economy.
(Writing by Justyna Pawlak; Editing by Mark Trevelyan and Cynthia Osterman)