Moscow and Washington are close to agreement on a formula that bridges differences over US demands to quickly re-impose UN sanctions on Iran if Tehran violates its commitments under a nuclear deal, officials have said.

Such an understanding would resolve a US-Russian dispute that threatened to scuttle an agreement meant to impose long term cuts on Iran’s nuclear programs in exchange for sanctions relief.

Washington sees a "snap-back" mechanism that allows previously lifted sanctions to be quickly reinstated as a cornerstone of any deal. Ben Rhodes, US President Barack Obama’s deputy national security adviser, told reporters last week that such a concept remained "the basic premise of our approach to sanctions."

But Russia opposes any automatic triggers.

A permanent member of the UN Security Council, it has vetoed attempts by the US and other Western council members to introduce such measures on other issues in the past. Opposed to such a precedent, it also rejects such a mechanism as part of the Iran nuclear deal now being worked on.

The two officials demanded anonymity because they were not allowed to discuss the confidential negotiations. They declined to go into details on the eve of a new round of Iran-six power talks on the expert level in Vienna and ahead of a June deadline for a deal.

But comments by France’s ambassador to Washington on Monday hinted at the possible compromise being worked on.

Gerard Araud said no structure was yet in place for snapping back sanctions but the basic premise would entail a majority vote of the five permanent Security Council members the US, Russia, China, Britain and France.

That would be at variance with the usual rule of consensus. Still, it could ease Russian and Chinese opposition to the "snap-back" principle.  Deputy Foreign Minister Sergey Ryabkov last month insisted that re-imposing sanctions "should not in any way be automatic" but would need to be voted on by the Security Council. Under the scenario mentioned by Araud, Moscow and its ally Beijing might be out-voted on re-imposing sanctions on Iran, but the issue would still go to a vote’s Russia’s central demand.

At the same time, such a vote could be held quickly, satisfying US demands of a "snap-back" a near immediate reinstatement of UN sanctions in case of Iranian violations.