The advances towards the provincial boundary between Raqqa and Deir al-Zor took place late on Saturday, the media unit, run by Damascus ally Lebanese Hezbollah, and the British-based Syrian Observatory for Human Rights said.
The army seized an oil field in the Sabkha area as part of the advance.
It was a rare advance for Damascus's forces in that area, which is close to territory controlled by the US-backed Syrian Democratic Forces (SDF), a Kurdish-dominated alliance separately fighting ISIS. It also brings government forces closer to Deir al-Zor province, another ISIS stronghold.
The Syrian army has active front lines with ISIS in western Raqqa province and has made recent gains there.
Separately in the north of Syria, one of the country's most complicated battlegrounds, Turkish-backed Syrian rebels are fighting US-backed Kurdish forces over control of some areas along the border, in clashes that threaten to distract from the fight against ISIS.
The Kurdish YPG militia, which forms the largest part of the SDF, controls much of northeastern Syria after capturing vast tracts of land from ISIS.
(Reporting by John Davison; Editing by Mark Potter)