The upbeat assessment and an agreement to meet again October 28 was a departure from the deadlock left by previous meetings over nearly two years.
At issue are suspicions outlined in reports from the UN's International Atomic Energy Agency that Iran worked secretly on trying to develop nuclear weapons, something Tehran denies.
As part of its probe, the agency is trying to gain access to a sector at Parchin, a sprawling military establishment southeast of Tehran.
The agency in turn said such an agreement would tie its hands by putting limits on what it could look for and whom it could question. It bases its suspicions of nuclear-weapons research and development by Iran on its own research and intelligence from the US, Israel and other Iran critics.
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The meeting was closely watched by the US and its allies as a test of whether Hassan Rouhani, Ahmadinejad's successor, was ready to deliver on promises that he sought to end Iran's nuclear standoff with the international community.
Its positive outcome was the latest in a series of encouraging developments along that line.
Both sides agreed to a new round of talks October 15-16 in Geneva, where Tehran will seek relief from crippling sanctions and the six nations will press Iran to scale back an atomic programme that could be re-engineered from peaceful purposes to producing weapons.