Mayadeen is one of the last strongholds of Daesh as it falls back upon the fertile area downstream of Deir Ezzor in the Euphrates valley and launches counter attacks in the central desert after losing swathes of territory this year.
“Units of our armed forces with the allied forces continue their advance on a number of fronts and axes in Deir Ezzor and its countryside ... and encircle Daesh terrorists in the city of Mayadeen,” the military source said.
The army and its allies reached Deir Ezzor in September after a months-long offensive across the Syria desert, and have in recent weeks pushed down the Euphrates toward Mayadeen.
However, counter attacks by Daesh in the central desert region have put pressure on the main supply road to Deir Ezzor from western Syria.
Syrian President Bashar Assad is backed in the war by Russia, Iran and Shiite militias including Lebanon’s Hezbollah, and its campaign against Daesh has mostly been on the west bank of the river.
A US-led coalition is also supporting a rival campaign against the jihadists, mostly on the east bank, where it is close to recapturing Raqqa, and has also advanced downstream to hold areas opposite Deir Ezzor.
US officials have previously said that Daesh had relocated some of its diminished command and propaganda structures to Mayadeen as it was forced from territory elsewhere.