BEIRUT (AFP) - The Islamic State in Iraq and Syria (ISIS) group launched a surprise attack near a town in eastern Syria they had lost six months ago, killing at least 25 members of regime forces, a monitor said on Thursday (April 19).
At least 13 militants were also killed in the attack which ISIS carried out near Mayadeen on Wednesday afternoon, the Syrian Observatory for Human Rights monitor said.
Mayadeen lies in the eastern province of Deir Ezzor 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 (SDF), 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 ISIS control.
Observatory director Rami Abdel Rahman said on Thursday morning that "(ISIS) attempts to advance in the direction of the town of Mayadeen are ongoing" from the Badia desert.
He said it was the "largest (ISIS) attack since they were expelled from the town" by regime forces and their allies in October 2017.
ISIS swept across large parts of Syria and neighbouring 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 ISIS has since lost most of it to a Russia-backed regime assault and a US-backed SDF offensive.
ISIS now control around 5 per cent 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 Ezzor 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.