Nearly 50 Assad regime soldiers and fighters of affiliated militia groups have been killed in separate attacks by Daesh and al-Qaida-linked Hayat Tahrir al-Sham (HTS) terrorist groups in different parts of Syria over the last two days.

The Syrian Observatory for Human Rights said Daesh militants ambushed regime forces in the Badia desert area of central Homs province Thursday night, setting off clashes that lasted for two days. The Observatory said at least 27 soldiers were killed, including four officers.

Daesh lost its last territories in Syria in March after months of battles with U.S-backed Syrian Democratic Forces (SDF), dominated by PKK-linked People's Protection Units (YPG) terrorists, in the eastern province of Deir el-Zour.

But the militants remain active in the desert to the west of Deir el-Zour, where they have taken refuge and increasingly targeted government troops and allied militia.

Liwa al-Quds, a pro-regime militia, said its fighters deployed to the desert area to liberate 500 soldiers besieged east of al-Sukhna town located northeast of the ancient city of Palmyra.

In a Facebook post, the militia said it successfully broke the siege and liberated the surviving soldiers, before pulling the bodies of those killed and damaged vehicles to safety.

Liwa al-Quds, one of the elite militias operating side by side with regime troops, didn't give a casualty figure.

Activists have reported a rise in clashes between regime forces and allied troops and Daesh militants following the end of the extremist group's territorial control in the nearby Deir el-Zour province.

The terrorist group, which once controlled large swathes of territory in Syria and Iraq, has kept a network of sleeper cells active in both countries.

It has also kept up its media operations. The Daesh-affiliated Aamaq news agency reported the attack east of al-Sukhna, adding that in 24 hours of clashes its militants killed nearly two dozen soldiers and officers. It said the militants also seized regime ammunition and vehicles.

Eight other regime soldiers were killed in a separate Daesh raid in the province of Deir el-Zour. At least six Daesh terrorist were killed during the attacks, according to the Britain-based group.

The attacks constituted Daesh's biggest military operation since losing its last holdout of Baghouz.

The vast Syrian desert, known in Arabic as the Badia, stretches all the way from the capital Damascus and the cities of Homs and Hama to its north to the Euphrates Valley near the Iraq border.

Meanwhile, an attack Saturday by a faction linked to Hayat Tahrir al-Sham killed 13 regime troops and allied fighters on the outskirts of the northern city of Aleppo, the Observatory said.

Heavy fighting and shelling broke out between both sides and the ongoing clashes have also taken the lives of at least eight members of the Abu Bakr Al-Sadeeq Army faction, Observatory chief Rami Abdel Rahman added.

The countryside to the west of Aleppo is the under control of HTS, which also controls the broader Idlib region.

That region is in theory protected from a massive regime military offensive by a deal inked in September by regime ally Russia and opposition backer Turkey.

But the accord has unwound as HTS took full control of the region in January after defeating rival opposition groups, and the area has come under increasing bombardment.

The Observatory said Saturday's attack came after regime bombardment overnight hit the western part of Aleppo province and the southeastern part of Idlib province.

A planned buffer zone around the region was never fully implemented as HTS refused to withdraw from it.

Assad's troops have managed to claw back some two thirds of the country since Russia intervened militarily in 2015, but Idlib remains beyond regime control.

On Friday, Assad insisted the main aim of the Idlib deal was to "eliminate terrorists," after they failed to pull out from the planned demilitarized area. He urged progress on removing "obstacles" to the stalled deal ahead of talks to be attended by Russia, fellow ally Iran, and Turkey next week in Kazakhstan.

Syria's war has killed more than 370,000 people and displaced millions since the conflict began with the repression of anti-government protests in 2011.