OH, to be a fly on the wall at the G20 summit in St Petersburg this week.

While the Syrian crisis is not officially on the agenda, the informal discussions that take place will doubtless shape the tumultuous events expected to unfold in the Middle East over the coming weeks. The latest escalation in Syria's war over the regime's alleged use of chemical weapons could not have come at a worse time in relations between Washington and Moscow.

Even before US President Barack Obama's arrival in the Russian city, the White House had cancelled a one-on-one meeting at the summit with Russian President Vladimir Putin in response to Moscow's granting of asylum to NSA leaker Edward Snowden.

On the sidelines of the summit -where the real talking will be done - Mr Obama is expected to meet with Russian human rights activists currently railing against the Kremlin.

This will only serve to further irk Mr Putin who is doing all he can to keep the diplomatic emphasis on the Middle East and pressure on Mr Obama rather than Moscow's domestic political woes.

While things are certainly frosty between the old Cold War rivals, there has at least been a few little conciliatory gestures from Moscow. The most significant of these has been its decision this week to suspend deliveries of S-300 air defence systems to Syria.

Some components have already made their way to President Bashar al Assad's regime but other key parts now being withheld by the Russians prevent it from becoming a fully operational weapon. Telling as Moscow's gesture over the S-300 air defence components is, it would be naive though to assume it represents a significant shift in Russia's position on Syria.

There is no getting away from the fact that should US military intervention commence and air strikes begin, the potential for some kind of regional blowback - as the intelligence community calls the unintended consequences of any military operation - would be profound indeed.

With respect to this, it is worth pausing to consider three players beyond Syria itself. What, for example, will be the response of Hezbollah, the Shi'a Islamic militant group and political party based in Lebanon that has been a key battlefield ally to the Syrian Alawite regime?

What, too, will Hezbollah's ­benefactor Iran do to support its proxy in the region? That in turn brings us to Israel, the one nation both Hezbollah and Iran would like nothing more than to see embroiled in a wider Middle East showdown.

Given they are inextricably connected, it is worth considering the respective roles of Hezbollah and Iran together.

According to the Beirut-based Daily Star newspaper, pro-­Hezbollah Sheikh Afif Nabulsi was recently quoted as saying "any (US) strike against Syria would be met by harsh responses against US interests in the region".

This of course could all be bluff on Hezbollah's behalf aimed at stoking up doubts and fears in Washington over the cost of intervention.

Should the threat be for real, however, and most likely it is, those responses would probably involve a wave of assassinations and hostage-takings in Lebanon, along with the targeting of other US assets and citizens in the region.

To that end, there have already been reports of talks between Iranian diplomats and Hezbollah leaders on such strategy in much the same vein as co-ordination between Hezbollah and the Damascus regime has operated over battlefield operations inside Syria itself.

According to the independent US-based intelligence think-tank Stratfor, Hezbollah has long had operatives capable of undertaking missions aimed at US interests in Lebanon and beyond, including in the US.

Some within the US intelligence community have even warned Hezbollah's operatives are better trained and more numerous than any group al Qaeda has ever controlled or deployed.

As for Iran itself, its rhetoric thus far over Syria has been relatively tame. That said, there is little doubt given Syria's role as Tehran's key regional ally, the survival of the al Assad regime will come second only to Iran's own in term of its leaders' and generals' priorities.

While Iran would find it difficult to directly retaliate against US interests, by using its proxies like Hezbollah, it is more than capable of creating mischief and striking back at the "Great Satan" as it likes to call the US when in belligerent mood.

This bring us to that other key Middle East player, Israel. Time and again, Israeli officials have privately stressed the country maintains its own specific red lines in the Syrian conflict. What Mr Obama defines as a red line is one thing, but as far as Jerusalem is concerned the al Assad regime's use of chemical weapons against rebels or Syrian civilians is not one of them.

Where Israel will have no ­hesitation in stepping up to the plate militarily, however, is if Syria should move towards transferring chemical weapons, advanced missile systems and air defence weaponry to Hezbollah, making it an even more formidable enemy than it already is.

Opening yesterday's G20 summit, President Putin suggested the Syrian issue should be consigned to a discussion over dinner rather than occupying time given to the main economic agenda. That will be some dinner indeed.

It will certainly give some of those attending a real dose of political indigestion.