"No one expected that one day would allow us to resolve international concerns about Iran's nuclear program, but I think today was a first step in what is bound to be a difficult process," the official told reporters during a background briefing.
The briefing came after more than seven hours of meetings among delegations representing Iran, the United States, Britain, China, France and Russia.
"This was a direct and candid discussion," the official said.
U.S. representatives expressed "serious concern" about the recent revelation of a secret nuclear-enrichment plant near Qom and made clear the importance of an inspection of the plant within the next few weeks by the International Atomic Energy Agency, the U.N.'s nuclear watchdog.
U.S. representatives, the official said, called on "Iranians to take concrete and practical steps consistent with its international obligations to build confidence and to demonstrate that its (nuclear) program is exclusively peaceful in nature," the official said.
The official also said Russia had agreed to enrich uranium, which would then be fabricated into fuel assemblies to be used in a nuclear reactor in Tehran. That, in turn, would reduce Iran's stockpile of low-enriched uranium, which some fear could be used for nuclear weapons, the official said. Iran agreed "in principle," with details to be ironed out at an Oct. 18 meeting in Vienna with IAEA officials.