Surprise IS attack 'kills dozens of regime soldiers' in east Syria
At least 13 militants were also killed in the attack which IS carried out near Mayadin on Wednesday afternoon, the Syrian Observatory for Human Rights monitor said.
Mayadin lies in the eastern province of Deir az-Zour on the western bank of the Euphrates River and is flanked by the vast Badia desert to its west and south.
A military source on Thursday however denied any attack against positions of the Syrian army along the western bank of the Euphrates.
But intermittent bombardment on army positions from its eastern bank had prompted retaliation with suitable weapons, the source said.
The US-backed Syrian Democratic Forces, an alliance of Kurdish and Arab fighters, control most of the territory to the east of the Euphrates, where a few villages are still under IS control.
Observatory director Rami Abdel-Rahman said on Thursday morning "IS attempts to advance in the direction of the town of Mayadin are ongoing" from the Badia desert.
He said it was the "largest IS attack since they were expelled from the town" by regime forces and their allies in October 2017.
IS swept across large parts of Syria and neighboring Iraq in 2014, declaring a cross-border "caliphate" in areas they controlled.
At its height their pseudo state covered an area the size of Italy, but IS has since lost most of it to a Russia-backed regime assault and a US-backed SDF offensive.
IS now control around five percent of Syria, according to Syria expert Fabrice Balanche.
But the militants have retained their ability to carry out deadly attacks. They hold pockets in Deir az-Zour and are present in the southern districts of the capital.
Since regaining full control of Eastern Ghouta to the northeast of Damascus from rebels last week, the regime has turned its attention to militant-held districts in the capital's south.
More than 350,000 people have been killed since Syria's war started in 2011 with the brutal repression of anti-government protests.