Russia, the United States and other world powers tried to put their sharp differences over Ukraine to one side on Tuesday as they kicked off the latest nuclear talks with Iran in Vienna.

The gathering is the second in a series of meetings aiming to transform by July a November interim deal into a lasting accord that resolves for good the decade-old standoff and removes the threat of war.

So far, despite disagreements over the Syria conflict and other issues, the six powers have shown a united front over Iran, but events in Ukraine in recent weeks have precipitated the worst crisis in East-West relations since the Cold War.

Following Sunday’s referendum in Crimea -- slammed as a sham by the White House and the European Union -- Brussels and Washington on Monday issued the first sanctions against a handful of Russian officials.

Already downbeat about prospects for a deal with Iran, Mark Fitzpatrick, a former U.S. State Department official now at the London-based International Institute for Strategic Studies, said the unfolding crisis made him “even more pessimistic”.

“The Russians will ... be less likely to make sacrifices for the sake of unity over the Iran issues,” Fitzpatrick told AFP. The Iranians, he said, “now have more reason to wait out the six powers”.

A senior U.S. administration official involved in the Iran talks said last week that diplomats “hoped that the incredibly difficult situation in Ukraine will not create issues for this negotiation”.

Even before the Ukraine crisis erupted, Russian President Vladimir Putin was reported to be discussing a major deal with Tehran whereby Moscow would get Iranian oil in exchange for money, goods and help in building new nuclear reactors.

This would undermine Washington’s efforts to cut off Iran’s main source of revenue -- a strategy which the U.S. credits with forcing Tehran to the negotiating table in the first place.

Mark Hibbs from the Carnegie Endowment for International Peace said this “huge barter deal” is a “carrot Moscow can dangle constructively to wrestle more concessions from Iran.”

“Or it can move forward unilaterally and damage the negotiation,” Hibbs told AFP. “Up to Putin to choose.”

Even without the spat over Ukraine, agreeing a lasting deal will be tough for Iran and the United States, China, Russia, Britain, France and Germany, known as the P5+1.

Under November’s agreement, Iran froze key parts of its nuclear program in return for minor sanctions relief and a promise of no new sanctions.

Although it could be extended, the deal is currently due to expire on July 20.

Iran has not permanently dismantled a single piece of atomic equipment. The bulk of U.N. and Western sanctions remain in place, depriving Iran of billions of dollars in lost oil revenues every week.

The six powers now want Iran to reduce permanently -- or at least for a long time -- the scope of its nuclear activities in order to make it extremely difficult for Tehran to develop nuclear weapons.

This would likely include Iran slashing the number of centrifuges “enriching” uranium -- which can be used for peaceful purposes and in a bomb, if highly purified -- and allowing tougher U.N. inspections.

In addition, Iran might have to change the new reactor being built at Arak to a kind that makes the extraction of plutonium -- the alternative to uranium for a bomb -- much more difficult.

But even though in return Iran would see sanctions lifted, it remains far from certain whether ultra-conservative elements in Tehran around supreme leader Ayatollah Ali Khamenei would accept such limitations.

Any deal that leaves some of Iran’s nuclear infrastructure intact would also be a hard sell to U.S. conservatives and to Israel, the Middle East’s sole if undeclared nuclear power.

“The final agreement will fall short of both sides’ ideals,” said Ali Vaez, Iran expert at the International Crisis Group. (AFP)