Islamic State militants have killed 21 Syrian regime fighters in an ambush as the group faces separate assaults on its last desert strongholds, a war monitor said on Tuesday.

The attack late Monday in Syria's southern province of Sweida came as US-backed forces advanced against the militants on the border with Iraq, said the Syrian Observatory for Human Rights.

It also comes with President Bashar al-Assad's forces poised to launch an attack on the northwestern province of Idlib, the last major region in Syria still controlled by rebels and extremists.

The IS ambush in Sweida's volcanic plateau of Tulul al-Safa sparked fighting that killed eight militants, the Britain-based Observatory said.

State news agency SANA reported heavy clashes with IS in the area, which lies some 100 kilometres (60 miles) southeast of Damascus, adding that government aircraft and artillery “targeted hideouts and positions” held by the group.

Government forces have been fighting IS in Sweida since militants carried out a wave of attacks in the mainly Druze province on July 25, killing 250 people according to the Observatory.

During their rampage, which targeted the provincial capital as well as rural areas, the militants also seized around 30 hostages, mostly women and their children.

At least 27 are believed to still be held, according to Human Rights Watch, after IS said it had beheaded a 19-year-old man and announced an elderly woman had died.

Observatory chief Rami Abdel Rahman said the hostages were believed to be held captive in the Tulul al-Safa area.

A source in Sweida told AFP that families had had no word of their kidnapped relatives in weeks.

IS has lost nearly all of the great swathes of territory straddling Iraq and Syria which it seized in 2014, but retains a presence in the vast Badiya desert that lies between Damascus and the Iraqi border, and holds a pocket in the Euphrates Valley in the east.

'Secret jails'

In that eastern pocket, a US-backed Kurdish-Arab alliance has for months been closing in on the town of Hajin east of the Euphrates River near the Iraqi border, and on Monday launched an assault to retake it.

In the early hours of Tuesday, the Syrian Democratic Forces alliance advanced inside the town, the Observatory said, with backing from the US-led coalition fighting IS.

“They have seized control of the northwestern part of Hajin” after residents fled, the monitor's chief Abdel Rahman said.

An SDF commander said the offensive on Hajin aimed to oust an estimated 3,000 militants, including a large portion of foreign fighters, from the town and surrounding areas.

“Most of the frontline commanders in this pocket are Iraqi,” said Ahmad Abu Khawla, a commander with the Deir Ezzor Military Council, which is part of the SDF.

After humanitarian corridors were opened to allow residents to flee the IS-held area, most civilians remaining inside were “directly linked to the group — hostages or the families of IS fighters”, he said.

Abu Khawla said IS had “secret jails where they hold civilians” captured in other areas of Syria.

Last year IS lost its de facto Syrian capital of Raqa in the north of the country, and this spring militants bussed out of the southern suburbs of Damascus.

Since Monday, 27 militants and 10 SDF fighters have been killed in the fighting for the Hajin pocket, the Observatory says.

More than 350,000 people have been killed and millions more displaced since Syria's war started in 2011 with the brutal repression of anti-government protests.