WASHINGTON, (Internet): The outcome of the standoff over Iran's nuclear programme may depend less on the results of Friday's presidential elections in the Islamic Republic, than on a decision to be made between now and September 1,500 miles to the northwest, in Moscow.

The Russian government has, since the election of President Barack Obama, been playing along with U.S. and European efforts to curb Iran's nuclear ambitions. But the consensus in Washington is that getting Russia to back and apply tougher economic sanctions against Iran will be much harder.

Moscow's stance on sanctions matters because no matter who wins Friday's election, both the frontrunners made clear that they had no intention of giving in to Western demands that Iran halt uranium enrichment.

The real decisions about Iran's nuclear future will be made by the clerical Supreme Leader, Ayatollah Ali Khamenei, who continues to defy the US, Europe and the International Atomic Energy Agency. So, Washington believes that coordinated international pressure is the only chance the West has to talk Iran back from the nuclear brink.

And Russia, which is helping Iran build its nuclear reactor at Bushehr and which is one of Tehran's key arms suppliers - as well as holding veto power at the U.N. Security Council - may hold the key to making tougher sanctions work.

The Administration's Iran policy czar, Dennis Ross, is preparing a package of sanctions to be adopted by the Security Council in the fall, that would target not just Iranian companies linked to Tehran's nuclear and missile programmes, but also its struggling energy sector. Unfortunately for the US, Russia is disinclined to go along with broadening the impact of sanctions.

"We should keep [sanctions] within the international non-proliferation efforts, and everything else that may be imaginable in other efforts goes beyond our goal," Russian deputy foreign minister Sergei Ryabkov recently told TIME.

Russia is certainly well placed to put the squeeze on Tehran because of its extensive business ties with Iran, including in the energy sector. For all its oil reserves, Iran's energy infrastructure is old and failing, and it is forced to import a large proportion of its gasoline needs.

Washington believes the regime will be responsive to moves by the outside world to blocks improvements and investment in its energy sector. Russian support is also key to achieving an international consensus for raising the pressure on Iran - if it signs on for new sanctions, the chances are much higher of achieving broad agreement at the UN in September.

Russia has been in a more cooperative mood since the advent of the Obama Administration, not least because the new President has been more responsive to Russia's own concerns on issues unrelated to Iran. "It is a sea of change in comparison with the previous administration, and we appreciate greatly this change," says Ryabkov.

"And we hope that the American administration values this engagement on our part on so many fronts." On Iran, the Russians won't rule out backing broader sanctions, and say they support using both carrots and sticks in dealing with Iran.