The Palais Coburg in Vienna is the venue for talks it's hoped with resolve the international row over Iran's nuclear ambitions.
Delegations from Iran and six world powers are hoping to make headway.
Iran's Supreme Leader is said to have given carte blanche for Iranian negotiators to provide guarantees the country's nuclear programme is peaceful.
The question now is whether the crisis over Ukraine will spill over because of the rift between Russia and the West.
The six world powers involved are the U.S. France, Germany, Britain, China and Russia.
Diplomats say there's no sign yet that the worst East-West confrontation since the Cold War will damage the search for a deal to end the Iran nuclear dispute.
There was no indication of such a spill over two weeks ago during expert-level talks.
Since then the U.S. and E.U have imposed sanctions on Russian and Ukraine officials over Crimean secession moves
