Two days of talks have begun with Iran to try to end a decade-long stand-off over Tehran's nuclear programme and avert the threat of a new war in the Middle East.
The nuclear armed US, Russia, China, France and Britain and Germany will push Tehran over its enrichment of uranium to 20% fissile purity.
A breakthrough is expected to be unlikely, with Iran expected to demand recognition of its right to enrich uranium for what it says is a purely peaceful nuclear programme.
The six countries are wary of letting diplomacy drag on without clear progress and giving Iran time to build up a programme which they fear is aimed at developing weapons.
New US and EU sanctions are due to come into force in two weeks, tightening economic pressure on Iran.
Yesterday, Israeli Vice Prime Minister Moshe Yaalon said without progress to ensure Iran does not develop nuclear arms, Israel "could find itself facing the dilemma of 'a bomb, or to bomb'."
EU foreign policy chief Catherine Ashton hopes to at least win assurances that Tehran is willing to discuss concrete solutions, opening the way to progress.
The six nations want a substantive response to their offer of fuel supplies for a research reactor and relief in sanctions on the sale of commercial aircraft parts to Iran.
Iran is also seeking an end to increasingly tough economic sanctions which have in recent months directly targeted its ability to export oil, its economic lifeblood.
At the last talks, in Baghdad last month, they asked Tehran in return to stop producing higher-grade uranium, ship any stockpile out of the country and close down the underground Fordow facility where such work is done.
A former Iranian negotiator, Hossein Mousavian, likened the powers' proposal to swapping "diamonds for peanuts", telling Reuters that the Moscow talks would probably fail without substantial concessions by the six powers.
