Much was made of the positive diplomatic noises that came out of the first round of talks on Iran's nuclear program in Geneva earlier this month. The spokesman for EU foreign policy chief Catherine Ashton's noted the novelty of "very detailed technical discussions," while the US praised a "level of seriousness and substance that we have not seen before."

Those positive noises were echoed by Tehran this week, after a meeting between Iranian officials and the International Atomic Energy Agency at its headquarters in Vienna, when Iranian ambassador Reza Najafi said, "I believe [that] with the submission of these new proposals by Iran, we have been able to open a new chapter of cooperation."

But while proposals and demands about uranium enrichment levels, sanctions relief, spot checks, and plutonium reprocessing are being passed back and forth behind closed doors, one huge but unmentioned issue is troubling both sides: the ongoing Syrian crisis.

The US praised the "seriousness and substance" of Iran's proposals in Geneva

'Iran's Vietnam'

The horrific conflict between Syrian President Bashar al-Assad and a wide array of rebels is a vastly more important issue for Iran that it is for the rest of the world. For the world's major powers, the conflict is one of the worst humanitarian disasters of the 21st century and a grave threat to stability in the Middle East, but for Iran, the war in Syria goes even deeper and represents a direct threat to its own power and internal order.

That much was made clear this week, when the magazine Foreign Policy unearthed evidence that Tehran is doing far more than merely training and financing Assad's militias. It is supplying the Syrian regime with discount oil and free loans to pay for it - a major sacrifice for a country under extreme economic pressure from international sanctions.

Ellie Geranmayeh, an Iranian policy analyst at the European Council on Foreign Relations, points out that this is partly a gesture of loyalty. "Iran has a historical alliance with the Syrian government, which was strengthened during the Iran-Iraq war, when Syria was one of its few allies in the region," she told DW. "Tehran hasn't forgotten that."

"Iran doesn't have many friends - most importantly, it has very, very few Arab friends, and Syria has been a lone Arab ally of Iran," said Shashank Joshi, analyst at the Royal United Services Institute in the UK. "It's important to understand that Syria is a link to Hezbollah, and therefore Iran's influence and power projection into Lebanon, and in turn its ability to apply leverage on Israel. The fall of Assad would mean being cut off from Hezbollah. But the irony is that given all the money it is spending, and all the military support, you could say Syria was Iran's Vietnam."

Iran has trained Assad's military in Syria

Hedging bets

With borders to states, like Iraq, Afghanistan and Pakistan, Iran is used to trying to maintain stability in a neighboring country teetering on the brink of collapse. On that point, Tehran's interests seem to converge with the world's major powers. Like the US and Russia, Iran has a reason to fear the rise of Sunni fundamentalism and Al-Qaeda inside Syria.

"Iran is making a fine calculation as it tries to maintain stability in the region, and it wouldn't be in America's interests for Iran not to play a role in Syria," points out Geranmayeh. "If you compare the situation in Afghanistan, Iran has played a significant role in bringing about security there and will continue these efforts even after a complete US withdrawal."

At the same time, Geranmayeh notes, Iran appears to be hedging its bets. As early as 2011, under former President Mahmoud Ahmadinejad, Iran hosted a conference to which it invited Syrian opposition groups, apparently showing an interest in speaking to them about the future of the country.

Impact on nuclear talks?

Given these common interests, could the Syrian crisis have any impact on the ongoing nuclear talks? For instance, would the US be willing to tolerate Iran's support for Assad in exchange for concessions on the nuclear program? Analysts are cautious about such deals.

"There is a common interest, but there always was a common interest," said Joshi. "Iran has also worked with the United States to help topple the Taliban. The nuclear diplomacy may have some spillover effect, but I doubt it will lead to grand bargains on all the disputed issues."

Any nuclear concessions do not mean Rouhani can concede ground on Syria

"I think it would be naive for us to think that Syria won't play a role in the nuclear negotiations, but the situation in Syria is so complex, with a plethora of actors involved, that I don't think there will be a concrete agreement - especially with the Syrian peace talks in Geneva approaching as well," said Geranmayeh. "Undoubtedly, they will discuss it, but Iran is just one of many actors shaping the Syrian peace talks."

Oliver Meier, a security analyst at the German Institute for International and Security Affairs (SWP), believes the two strands of diplomacy are being kept strictly divided. "My impression is that there is a deliberate attempt to keep these issues separate and not have them interfere in the nuclear negotiations," he told DW. "It would be potentially conducive to the nuclear talks if it was possible to involve Iran in the Geneva talks on Syria - there is that link in the background. But in terms of the actual talks on the nuclear program, I think they will try and stay away from Syria."

Joshi also thinks Iran has plenty of incentives not to bring up Syria in the nuclear negotiations, "The more advanced the nuclear diplomacy gets - and it's going quite well I would say - the harder it is to progress on other issues, because President Rouhani doesn't want to look like he is giving in to Iran's enemies," said Joshi. "And anyway, those other areas, particularly Syria, are controlled by elements of the Iranian state that are more hard-line."

Iran's internal affairs complicate all the speculation. "Iran is a fractured state with many parts - you have actors like the Revolutionary Guard that have a lot of sway over Syria policy," said Joshi. "So even if Rouhani reduces nuclear tensions, it doesn't mean he's able to then make concessions on Syria."