Russia, the United States and other world powers put their sharp differences over Ukraine to one side on Tuesday as they held their 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 secession referendum in Crimea -- slammed as a sham by the White House and the European Union -- Brussels and Washington on Monday issued sanctions against a handful of Russian officials.

On Tuesday Russian President Vladimir Putin signed a treaty claiming Crimea as Russian territory and said the Black Sea peninsula has always been "in the hearts" of his compatriots.

Despite the tensions, a spokesman for Catherine Ashton, the powers' chief negotiator and EU foreign policy chief, said he had seen "no negative effect" on the Iran talks, with the six "still united".

He said later that the negotiations, set to continue Wednesday, were "substantive and useful". Late Tuesday the US and Iranian delegations also held a bilateral meeting.

But 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".

Even before the Ukraine crisis erupted, 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.

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."

- A lasting deal -

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 programme in return for minor sanctions relief and a promise of no new sanctions for six months.

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

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 but also in a bomb, if highly purified -- and allowing tougher UN inspections.

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 sceptical US lawmakers and to Israel, the Middle East's sole if undeclared nuclear power.

Iran Foreign Minister Mohammad Javad Zarif said in an opinion piece to appear in Wednesday's Financial Times that both sides need to "show courage -- far more than has been displayed so far".

He wrote that Iran's "counterparts will have to make tough choices. They will have to back up rhetoric with action."