At least three Russian soldiers have been killed in a firefight in Syria’s de-escalation zone this week, the Vedomosti business daily has reported Thursday, citing an unnamed military source.

Syrian President Bashar al-Assad’s forces, backed by Russian airpower, have been waging an offensive in the Idlib region, the last remaining rebel-held territory in Syria. Much of the region is controlled by jihadists linked to the former Nusra Front, which was linked to al Qaeda.