Vienna: Tehran may have a new ally on Tuesday when Iran nuclear talks reconvene — the Ukraine crisis. US-Russian tensions over Ukraine could fray the search for consensus on what Iran needs to do to ease fears it could make atomic arms.

Both Washington and Moscow are emphasising that their commitment to eliminating any Iranian proliferation threat overrides their clash over Ukraine.

But diplomats tell The Associated Press that Moscow and Washington are wide apart on how much Tehran needs to trim its nuclear programme, a split that Iran could exploit.

Ahead of the resumption of the Vienna talks, US State Department spokeswoman Jennifer Psaki said she expects Russia to “remain an active partner” in the attempt to persuade Tehran to agree to long-term nuclear curbs in exchange for full sanctions relief.

A senior Russian diplomat said his country would work “actively” to reach a deal. He demanded anonymity because he did not have permission from Moscow to comment.

But former US nuclear negotiator Gary Samore says any superpower tensions will make Tehran feel “under much less pressure to make concessions.”

That, in turn, could affect what the US sees as central goal - reducing the number of Iranian centrifuges set up at uranium enrichment sites.

Iran says its enrichment programme is meant only to make lower-enriched fuel for reactors, scientific research or for medical treatments. But because enrichment to very high levels creates weapons-grade uranium used in nuclear warheads, Washington wants Tehran to scale back from nearly 20,000 centrifuges to no more than a few thousand.

Russia’s demands are far less strict. Two diplomats told the AP that Moscow was open to Iran keeping many more of the machines — perhaps even the status quo of the nearly 20,000 — with further negotiations on how many would be allowed to operate.

Moscow’s condition would be that Iran ratify an agreement with the UN nuclear agency that would give agency experts wide-ranging inspection powers to make sure Tehran’s nuclear programme is peaceful, they said. The diplomats are familiar with the details of the closed-door talks but demanded anonymity because they were not authorised to discuss them.

China usually supports Russia’s position at the negotiations, but Moscow’s stance is rejected by the US, Britain, France and Germany.

Moscow and Washington have been able to bridge previous differences over Iran. And former State Department official Mark Fitzpatrick notes that even during the Cold War, the two nations cooperated “despite many periods of intense distrust”.

Fitzpatrick, who is now with the International Institute for Strategic Studies, lists Russian-US teamwork on bringing about the nuclear Nonproliferation Treaty and extending its reach as well as concluding several arms control agreements as examples of how common interests can override even the worst tensions.

Still, he says a Moscow at odds with Washington over larger geopolitical issues could put its own agenda — including building nuclear reactors for Iran — above cooperation. Russia, he says, always “marches to its own drum.”

Samore, of Harvard’s Belfer Centre, says Iran is bound to feel “emboldened” — and public statements from Tehran seem to mesh with that view.

At the same time Moscow is negotiating on reducing Iran’s nuclear programme, it is in talks on expanding it by building new reactors there. Iranian Foreign Minister Javad Zarif said last week his country and Russia had common interests, and that Tehran was banking on “Moscow’s help to reach the final agreement.”

Iran’s Fars news agency, meanwhile, appeared to take a dig at veiled US threats of military strikes if negotiations fail and Washington believes that Tehran is working on a bomb.

It republished a cartoon showing President Barack Obama peering into an empty paint can marked “Red Line.”

The cartoon shows a grinning Russian President Vladimir Putin walking away, with the caption: “I think you used it all on Syria.”