BEIRUT – Iraq’s national security adviser met with Syrian President Bashar al-Assad last month to explore the prospect of direct military cooperation between their forces in the fight against the so-called Islamic State (ISIS).

Iraqi warplanes have been bombing ISIS positions in Syria since February, and it remains unclear what bolstered military cooperation “on both sides of the border” could entail.

However, over the past two weeks, Iranian proxies in both Syria and Iraq made significant advances toward their respective sides of the border, suggesting an intent to join forces to battle ISIS in Syria – a move that could give Assad unprecedented leverage in an upcoming offensive on the militant-held eastern city of Deir Ezzor.

The prospect of joining forces became more likely on Monday after the Popular Mobilization Forces (PMF), an Iranian-backed Shiite militia working with the Iraqi army to fight ISIS, reached the Syrian border.

While recent gains sparked a conversation about cooperation, it is likely to be weeks – perhaps months – before the Iran-backed forces, who report to the governments in Damascus and Baghdad, could actually link up across the frontier.

A Race to the Border

Following Monday’s advance in Iraq, a PMF spokesman said his forces are ready to march into Syria to battle the militants, but added that they would need permission from the Iraqi and Syrian governments before they could do so.

PMF Deputy Commander Abu Mahdi al-Muhandis, speaking to Iraqi al-Ahad TV, called the cross-border cooperation “a natural right for Iraqi military forces, including the PMF… to defend the security of Iraq… from the sources of terrorism outside its border.”

On the other side of the frontier, Iranian proxies in Syria have been advancing toward the Iraqi border on two fronts for more than a couple of weeks. Tehran-backed paramilitary groups began mobilizing around a U.S. training camp for anti-ISIS rebels in the southern Syrian town of Tanf, near the Iraqi border, in mid May – a move that has put Assad’s allies in close confrontation with U.S.-backed forces on the ground.

Videos released on social media purported to show fighters from the Iraqi Imam Ali Brigade preparing for the battle on Tanf. Other Iranian-backed Iraqi militias such as Harakat al-Abdal and Kata’ib Sayyid al-Shuhada are also operating in the area, according to Phillip Smyth, a researcher at the University of Maryland who focuses on Shiite militarism in the Middle East.

Additionally, hundreds of fighters from the Lebanese Hezbollah have reportedly deployed to fight U.S.-backed rebels in Tanf, according to reports by Fars News Agency, which is affiliated with Iran’s Revolutionary Guard Corps.

On the second front, Syrian troops and affiliated forces are pushing northeast from the ancient town of Palmyra toward the ISIS-controlled crossroads of Sukhna, which lies more than 124 km (77 miles) west of the militants’ new de facto capital Deir Ezzor. They are also pushing southeast of Palmyra toward the Iraqi border, into areas currently held by ISIS.

“It is necessary to understand that this border is very important to the Syrian regime and its allies,” Fabrice Balanche, a visiting fellow at The Washington Institute, told Syria Deeply over Skype, explaining that control over the frontier would give Assad a boost against ISIS, by isolating the group into two separate pockets across the border and cutting off its supply lines from Iraq.

The influx of Iranian-backed paramilitaries into Syria from Iraq would also bolster the Syrian government’s combat power as it pushes toward the eastern ISIS stronghold of Deir Ezzor, giving the Assad regime unprecedented leverage in the fight against the militant group.

According to Balanche, a ground link across the border is also especially important for Tehran, since it would provide a vital supply route for Iranian weapons into Syria and is crucial to Iranian plans to construct a ground corridor from Tehran all the way to the Mediterranean coast.

The Long Road Ahead

Though significant, recent gains by both Syrian and Iraqi paramilitaries near the border do not yet allow Iranian-backed forces in Syria and Iraq to join ranks. “We are still at the beginning of a process that began only one month ago,” Balanche said.

Pro-government forces and the PMF can not link up in border positions captured by the Iraqi paramilitary group earlier this week, since these areas lie adjacent to Kurdish-held territory in northeast Syria.

The Syrian Democratic Forces, a U.S.-backed alliance of Kurdish and Arab fighters which controls the region, will not allow pro-government forces to enter its territories nor will it make way for Iraqi paramilitaries to use Hassakeh as a gateway into the rest of the country.

An SDF commander said Wednesday that his forces would fight Iraqi Shia militants if they attempted to enter territory they controlled in Syria. “If Hashd forces attempt to enter our areas, our forces [SDF] will fight them,” Talal Silo said, referring to the PMF.

On the Tanf front, the presence of U.S.-backed rebels known as Jaysh Maghawir al-Thawra, as well as U.S. and British Special Forces, who are preparing opposition groups for upcoming battles against ISIS, has complicated attempts to reach the border.

U.S. warplanes carried out a series of attacks on a convoy of Iranian-backed paramilitary groups, including the Imam Ali Brigades, approaching the base on May 18, forcing pro-government militias to retreat north toward the Zaza checkpoint and Sabaa Biyar, a small town near the Damascus-Baghdad highway.

U.S.-coalition warplanes also dropped leaflets in the Tanf region vowing to retaliate against any future government advances against its forces.

According to the Syrian Observatory for Human Rights, a U.K.-based monitoring group, the leaflets said: “Any moves towards al-Tanf is considered hostile and we will defend our forces… You are within the safe area, leave this area now.”

The U.S. is also bolstering its combat power in southern Syria to fend off the threat of Assad’s Iran-backed allies, a spokesman of the U.S.-led coalition said Thursday.

In effect, the U.S. has created a buffer-zone between the Iraqi and Syrian borders, in a move that some claim is part of Washington’s effort to carve out an area of influence in southern Syria.

This leaves pro-government forces with only the southeastern route from Palmyra toward border areas roughly 100 km northeast of Tanf, where Iranian-backed groups could link up with PMF forces traversing down the frontier toward areas in Iraq’s western Anbar province. However, this push will likely take time and effort, especially since ISIS still has forces deployed across the vast desert territory separating pro-government forces and the Iraqi border.

“The viability of linking up across the border, it’s still somewhat off,” Aymenn Tamimi, a research fellow at the Middle East Forum, a U.S.-based think-tank, told Syria Deeply. “A real linkup would require much more substantial gains for the regime and its allies pushing east.”