Syrian regime troops have seized more territory from the Islamic State jihadist group in a central desert region, almost entirely surrounding its fighters there, a monitor said Friday.

Backed by Russian warplanes and pro-government militias, the Syrian army has made considerable progress in its months-long offensive to retake the Badiya.

The vast desert region, which stretches from the country's centre to the Iraqi and Jordanian borders, has been held by IS since 2014.

"The regime managed to encircle the area of Okayrbat and the surrounding 44 hamlets, straddling the central provinces of Hama and Homs," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

Syria's official SANA news agency also reported that Okayrbat had been surrounded.

Russia's defence ministry said the Syrian army had taken control of the last supply route for arms, ammunition and equipment in the area.

It said the jihadists were seeking to flee towards their stronghold of Deir Ezzor to the east.

"The Russian air force is continuously conducting reconnaissance drone flights drones to find and destroy armoured vehicles, pickup trucks with heavy weapons and cars used by the terrorists," it added.

Regime troops have also seized four gas fields in the central desert, the British-based Observatory said.

Victory over IS in the region is seen as key to the army's hopes of retaking Deir Ezzor, the last Syrian province that remains nearly completely under IS control.