If the United States is wary of Russia’s role, it has little alternative to involving the country. To expand the time that Tehran would need to build an atomic bomb, it is critical to remove from Iran a substantial amount of the 28,000 pounds of uranium that the International Atomic Energy Agency recently estimated it had produced.

The larger obstacle to reaching an agreement on the uranium may be the Iranians, the senior official said, because “what is less certain is whether Iran will accept the reasonable proposals” on the table, or “will continue to make excessive demands that are not aligned with its practical nuclear needs.”

When Secretary of State John Kerry arrives in Oman this weekend to resume negotiations with his Iranian counterpart, he is expected, among other things, to negotiate over caps that would limit how much uranium Iran could keep on hand at any given time.

But history suggests that an agreement with Iran to part with much of its nuclear stockpile, which it has spent billions of dollars to amass despite Western sanctions and sabotage, is never a sure thing. A deal struck between the Obama administration and Iran in 2009 to ship some of its nuclear fuel out of the country — an agreement that would have left Iran with less than the required amount of uranium to make a single nuclear weapon — fell apart when it was brought to Ayatollah Ali Khamenei, the supreme leader. There is clearly a concern that the same thing could happen again, or that the Revolutionary Guards Corps, which is believed to run the military side of the nuclear program, could object.

And for the Iranians, Russia’s involvement is not necessarily a plus.

“There have been numerous iterations of Iranian-Russian cooperation in the past, and they have not come to fruition,” Karim Sadjadpour of the Carnegie Endowment for International Peace said on Sunday. “Often the economics do not make sense. And the Iranians mistrust the Russians almost as much as they mistrust the United States.”

Russia’s calculus is also complex. It stands to gain financially from the deal, but it also has an incentive to see the nuclear standoff between Iran and the rest of the world continue, because an embargo keeps Iranian oil off the market. With oil prices falling, a flood of exports from Iran could further depress prices.