Nascent nuclear talks between Iran and western powers are kindling some fears that after years of being sidelined from global diplomacy, Tehran may once again find a seat at the table.

US Secretary of State John Kerry has stressed that despite burning issues such as Syria and Afghanistan, the focus has so far remained on reining in the Islamic republic's suspect nuclear program.

"We're not in a larger discussion. We're not having a geopolitical conversation right now," Kerry insisted recently.

And State Department spokeswoman Jen Psaki on Tuesday said Washington's view of Tehran as the world's leading sponsor of terrorism had not changed.

Yet the unease being voiced by Washington's traditional allies such as Saudi Arabia and Israel reveals the deep-rooted skepticism that these tentative overtures could lead to a broader rapprochement with an Iranian leadership, viewed by many as a threat and an outright enemy.

On Tuesday, British Prime Minister David Cameron went a small step further in rehabilitating Iran back onto the world stage, when he telephoned new Iranian President Hassan Rouhani ahead of the Wednesday's next round of nuclear talks in Geneva in the first such high-level contact in more than a decade.

But despite the wariness, there's a recognition too that Shiite Muslim Iran could be key to resolving other issues bedevilling global politics.

"If there is a resolution to the nuclear crisis, I do believe it will open up avenues for at least discussing other issues in Syria, Afghanistan and even perhaps cooperating on some of these issues," said analyst Alireza Nader, senior policy analyst with the RAND Corporation.

On a conference call with reporters, he pointed to past cooperation between Iran and the US on forming the new government which emerged after the 2001 ousting of the Taliban in Afghanistan -- efforts in which new Iranian lead negotiator and foreign minister Mohammed Javad Zarif played a key role.

But whether Iran should be invited to join a long-planned peace conference bringing together the Syrian regime of President Bashar al-Assad and the opposition has emerged as a major bone of contention, hampering even the convening of the talks dubbed Geneva II.

Iran, which has not had full diplomatic ties with the US for more than three decades, stands accused of propping up the Assad regime and prolonging the bloody conflict, which in nearly three years has now claimed some 120,000 lives, by sending the Syrian leader weapons, money and military advisors.

The US has insisted that to win a place at the negotiations, the Iranian leadership must first sign up to a 2012 Geneva communique which called for a transitional governing body in Damascus to guide the country towards new elections.

Turkish Foreign Minister Ahmet Davutoglu said Monday in Washington that he had discussed a "political settlement" to the Syrian conflict when Zarif visited Turkey recently.

"In principle, we are not against Iran playing any role in any process... We want all regional actors to be on board," Davutoglu, who visits Iran next week, told reporters.

"But of course, all those who are coming to Geneva II must commit themselves not to support the regime doing these massacres."

He signalled that he believed "there are certain indications that Iran is trying to have channels of communication for a new seat of power to resolve the Syrian crisis."

RAND analyst Nader agreed Tehran was not necessarily "committed to the specific person of Assad as Syria's leader."

"What the Islamic republic wants is to maintain its influence, not just in Syria, but the broader Levant and Lebanon specifically, for Iran to be a regional power and not completely lose its influence to its main competitor, Saudi Arabia."

Michael Doran, senior fellow for the Brookings Saban Center for Middle East policy, warns of "two warring axes" amid the upheavals of the Arab spring -- a horizonal axis of Iran, Syria and the Iran-backed Lebanese Hezbollah militia, and a vertical one with Sunni-majority Saudi Arabia, the Gulf, Jordan and Turkey.

"The two axes intersect violently in Syria," Doran writes.

"For America's allies the conflict in Syria is a zero-sum game, the defining battle for the future of the regional order. Much to their consternation, however, Washington refuses to take a side."

Analysts say a re-emergent Iran could leave Washington with a delicate balancing act.

"An estrangement for more than three decades is beginning to thaw, that changes power dynamics across the Middle East and the United States is going to have different relationships potentially as a result of this," said Joel Rubin, director of policy from the non-governmental Ploughshares Fund.

Much like the United States had to handle its rapprochement with China and the collapse of the Soviet Union, it will once again have to navigate a changing world order, Rubin said.