As rebels capture territory in northern and western Syria, including the cities of Aleppo and Hama, other conflicts are playing out in the east.

The region, stretching from the central desert to the far reaches of the border with Iraq, is home to Syrian government troops, Iran-backed militants and pockets of ISIS fighters and Kurdish militias backed by the US. The groups have fought against each other in recent days, as the lightning advance of rebels in the west of the country remains in the spotlight.

"ISIS disappeared in the past week, but today they started moving in the Syrian desert. In a limited way, they put up their checkpoints there and were moving around in the desert with greater ease," Mr Mohammed Hassan, a Syrian analyst from Deir Ezzor, told The National.

ISIS members were on the move in Al Kum area, south of the city of Raqqa, he added. Within a few hours, the Kurdish-led Syrian Democratic Forces said it had taken “comprehensive and controlled measures” to counter ISIS threats and “counter any attempt by ISIS to expand into our areas". The SDF controls territory to the north of where ISIS has operated.

Although the terrorist group was defeated in Syria in 2017, it has remained active in small pockets of central Syria. ISIS now appears to be taking advantage of the rebel advance across the country, which has forced the Syrian army and its Iran-backed allies to leave cities including Hama.

"What helped ISIS advance in this period is that the Syrian regime started to move its forces located in bases in the Syrian desert towards cities around Raqqa and Deir Ezzor, and from there it needs to move reinforcements to Homs and Damascus, in an attempt to prevent any attack from the Syrian opposition," Mr Hassan said.

The US-backed SDF has also seized the opportunity to expand its sphere of control. On Tuesday, the group’s Deir Ezzor Military Council said it had taken control of seven villages on the eastern bank of the Euphrates that had been under the control of government forces and allied Iran-backed militants since the collapse of ISIS.

“This has been a long-time objective of the SDF and, with the significant changes of control elsewhere in Syria and the general confusion among regime-aligned forces, they are seizing the opportunity,” Broderick McDonald, an associate fellow at the International Centre for the Study of Radicalisation at King's College London, told The National.

The SDF is under pressure, though. The force is led by the People's Protection Units (YPG), a Kurdish group linked to the Kurdistan Workers’ Party (PKK), which has fought a decades-long insurgency against the Turkish state. The YPG and PKK are considered terrorist organisations by Turkey.

In an operation called “Freedom Dawn", Turkish-backed fighters have advanced to areas west of Lake Assad on the Euphrates. Tens of thousands of people are reported to have fled the areas towards Kurdish-held territory farther east, while Turkish-backed groups have filmed people welcoming them.

The rebels may attempt to take control of areas on the other side of the river, said a senior Syrian opposition politician, adding that Kurdish forces needed to cut ties with the PKK.

“As for the other areas where there is SDF, like Manbij and the east of the Euphrates, we believe they will face great pressure there,” Ahmed Touma, head of the Syrian opposition delegation at peace talks held between Turkey, Russia and Iran, told The National.

“We said to them, time and time again, that our hand is extended for peace, but on the condition that you sever your association with Qandil,” he added, referring to the PKK base in the mountains of northern Iraq.

Farther east, the SDF’s ability to hold territory depends on policy changes that may occur when US president-elect Donald Trump takes office in January. If he decides to withdraw US troops from Syria, that would weaken the SDF, making it easier for Turkish-backed fighters to advance into SDF territory, or force it into a compromise.

“The SDF is seeking to cement its control over remaining pockets in Deir Ezzor before widely expected changes in policy from the incoming Trump administration that could see the withdrawal of all or most US forces in Syria,” Mr McDonald said. “More broadly, each actor in Syria is now actively seeking to consolidate territory that will put them in a stronger position before any future political negotiations.”

Iran-backed militants also have a significant presence in eastern Syria around Deir Ezzor, where they helped to secure control of the western bank of the Euphrates for President Bashar Al Assad's government, after the collapse of ISIS. About 900 US troops remain in Syria as part of anti-ISIS operations, but they also carry out what the US describes as self-defence strikes against Iran-backed militants.

On Tuesday, US Central Command forces said it destroyed rocket launchers, vehicles and mortars after militants fired towards American forces at a base in eastern Syria. Pentagon spokesman Maj Gen Pat Ryder said it was not clear who used the weapons, but Iran-backed groups and Syrian government forces are known to be in the area.

Iranian-backed forces, spread across other parts of government-held territory, as well as eastern Syria, have not yet moved en masse to prevent the government from losing territory to the rebels in the north and west. Iran-backed groups, including Hezbollah and parts of the Iraqi Popular Mobilisation Forces, are widely reported to use Syria as a conduit for weapons, supplies and fighters, especially in near the border with Iraq.

Israel has sought to combat that by carrying out dozens of strikes, although it rarely acknowledges them. The attacks, especially in the past year, have limited the Iran-backed groups’ ability to move around Syria, including to support government troops.

“There are significant Iranian-backed forces in eastern Syria, but so far the response from Iran has been slow,” said Mr McDonald. “This is likely to change as Mr Al Assad is now in a very dangerous situation, but it did not come fast enough for the regime and Iran to stop the rebels from capturing Aleppo and Hama.”