In contrast, Iran views the Islamic State (IS) as the most imminent threat to regional security, and more broadly opposes the US military presence in its neighborhood. A senior Iranian official who spoke with Al-Monitor without attribution said Syrian President Bashar al-Assad’s state security apparatus , Hezbollah and Russian military assistance provide the most effective means to counter these threats. But for Saudi Arabia, IS and Sunni radicalization are a derivative of the alienation being fueled by the activities of Iranian-commanded Shiite militias in Iraq and Tehran’s backing for Assad.

First, there is critical divergence over the source of instability in the Middle East, and therefore stark contrast in conceptualizing the “enemy.” For Saudi Arabia, Iran is a “ nefarious ” regional actor that has overstretched its ambitions in the Arab world, bringing with it great instability. In a recent off-the-record conversation with a senior Gulf Arab official, it was clear that Saudi Arabia’s regional priority was focused on limiting Tehran’s orbit of influence in Syria and consequently weakening its stronghold in Lebanon.

In the absence of buy-in from both Riyadh and Tehran, the Syrian conflict remains deadlocked. This stalemate is borne out of three fundamental areas of divide in their respective maximalist positions.

There had been hopes that this month’s UN General Assembly in New York could provide the platform to actively pursue diplomacy on Syria. But UN Special Envoy Staffan de Mistura has so far been unable to effectively set into motion plans for "de-conflicting" Syria through the formation of a new contact group.

The opening created in the aftermath of the July 14 nuclear deal between Iran and the six world powers (the five permanent members of the UN Security Council plus Germany, or P5+1), which could have triggered a diplomatic uplift over Syria, has so far been wasted. Instead, the regional divide over Syria is deepening and major powers have failed to seriously press Riyadh and Tehran to de-escalate their proxy war in Syria. Meanwhile, the costs for the West and the Middle East continue rising as a result of the festering crises in Syria and Iraq.

A second hurdle for diplomacy is that Saudi Arabia and Iran cannot reach consensus on the starting point for ending the conflict in Syria. In principle, both agree on the need for the formation of a transitional government, as outlined by the Geneva 1 conference. However, Riyadh sees no place for Assad in this transition. In contrast, a senior Iranian official who recently spoke to Al-Monitor on condition of anonymity said that recent gains made by IS have made it essential that Assad have a role in the transitional government in order to prevent the dissolution of Syria’s security apparatus and a total state collapse.

Since the nuclear deal, European and US officials have privately acknowledged that a shift toward a middle position may be necessary. British Foreign Minister Philip Hammond suggested last week that a time-limited role could be carved out for both Assad and his political circle as part of the transitional government. On Sept. 18, US Secretary of State John Kerry similarly suggested that the United States is open to the possibility of Assad remaining in power for a short duration under a peace settlement. Such proposals have so far been swiftly rebuffed by Damascus. Both Saudi Arabia and Iran are equally unlikely to openly welcome this approach.

There is also sharp divergence on post-Assad Syria. A senior official from a Gulf Cooperation Council state outlined during a recent private meeting that Saudi Arabia would support a central government in Syria that operates under a power-sharing system between the Sunni majority and minority groups. In a separate conversation, a senior Iranian official who spoke without attribution argued that such a system was unworkable, given that Syria’s largely secular society did not lend itself to proportional power-sharing of the kind seen in Lebanon or Iraq. The official further outlined that Iran places little faith in Sunni Islamist factions offering protection for minorities, who have largely fled such opposition-held areas to seek refuge in government-controlled areas.

A third factor stalling diplomacy is the ratcheting up of military support to opposing warring factions in Syria by external powers. Saudi Arabia’s strategic cooperation with Turkey and Qatar this year has strengthened the anti-Assad “Conquest Army” insurgency in the north and south of Syria, delivering severe blows to regime-held territory. According to a senior Gulf Arab official who spoke without attribution, these operations will continue until Assad is substantially weakened, to force a shift of position from Iran and Russia.

In response, Iran and Russia have noticeably increased their military assistance to Damascus. Israel alleges that Iran is orchestrating Hezbollah’s expansion into the Syrian-controlled sector of the Golan Heights to consolidate its future strategic positioning against Israel. Meanwhile, the Pentagon suspects that the recent build-up of Russia’s military presence in Syria’s northwestern Latakia province paves the way for Russia to boost its forward air operating bases in Syria.

Russia’s move has been open to different interpretation. For some, it has raised alarm bells that Assad’s supporters are willing to escalate the conflict at the expense of a diplomatic track. Others see this as an indication that Russia is bolstering the regime in advance of a political settlement and is open to coordination with the West in combating IS. The announcement by the US administration on Sept. 18 that it is prepared to cooperate with Russia on military efforts in Syria lends support to the latter view.

High-level talks are due to take place on Sept. 21 between Iran and Russia to discuss Iran’s “four-point plan” for Syria. A senior Iranian official who spoke on condition of anonymity told Al-Monitor that while the details of the plan were still a work in progress, the first stage aimed to contain and deplete IS, with the second stage focused on reaching consensus over the type of government in control of Damascus and Assad’s future role.

According to an Iranian analyst involved in Track II diplomacy, Tehran is also exploring options to cut a deal with Turkey to exert pressure on IS and remove backing for other Syrian opposition forces. Turkish President Recep Tayyip Erdogan has faced considerable domestic backlash over his Syria policy, and this could pressure him to find a middle way out of the conflict. Turkey’s foreign policy priority has also seemingly shifted from toppling Assad to fighting the Kurdistan Workers Party.

As with the Iranian nuclear file, resolving the deadlock over Syria is a global security issue that will require top-heavy and persistent diplomacy. The refugee dilemma facing the West is worsened by the regional polarization between Iran and Saudi Arabia that looks set to prolong the Syrian conflict. Since the nuclear deal was struck, the West has been more hesitant than before in siding too closely with either side. This is perhaps the most prudent stance as neither the current Iranian nor Saudi models for Syria can confidently deliver a sustained cease-fire, protection for minorities or an effective anti-IS campaign.

It is clear that a workable political track in Syria can only ultimately emerge as a compromise between the road maps offered by Saudi Arabia and Iran. Given the West’s military fatigue and precedent of failed intervention in Iraq, the most realistic option is for Western powers to commit to intensified diplomacy that pushes Tehran and Riyadh closer to a settlement, rather than becoming bystanders to escalating tensions in Syria.