Russia, the US 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.

World powers have not seen any impact so far from tension with Russia over Crimea on nuclear talks with Iran, a spokesman for the European Union's foreign policy chief said on Tuesday.

"I haven't seen any negative effect," spokesman Michael Mann told reporters. "We continue our work in a unified fashion".

Already downbeat about prospects for a deal with Iran, Mark Fitzpatrick, a former US 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 US 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 US 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 said. "Up to Putin to choose."

AFP - Reuters