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.
Today 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".
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".