Their comments enforce concerns that obstacles to a pact remain beyond the public debate on how far Iran must open its nuclear program to outside purview under any deal.
Iran's supreme leader, Ayatollah Ali Khamenei, has for weeks rebuffed US demands that UN nuclear monitors have access to military sites and nuclear scientists as they monitor Tehran's commitments under a deal and probe allegations of past work on atomic arms.
Negotiators have been meeting five days a week in Vienna over the past few weeks. The two diplomats are familiar with the progress of the talks and spoke shortly before a planned five-day round reconvened today. They demanded anonymity because they are not authorized to discuss the confidential negotiations.
Ways of implementing specific parts of the deal are supposed to be contained in four or five annexes to the main text of an agreement.
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Both sides remain publicly committed to June 30. Still, the diplomats said all nations at the table recognize that a delay up to July 9 is not a deal-breaker.
If US Congress receives a deal by July 9, it has 30 days to review it before US President Barack Obama could suspend congressional sanctions. Postponement beyond that would double the congressional review period to 60 days, giving both Iranian and US opponents more time to work on undermining an agreement.
Tehran denies any interest in or work on atomic arms, but wants negotiations with the United States, Russia, China, Britain, France and Germany to conclude with an end to sanctions imposed over its nuclear program.