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Local sources told °®Âþµº the Russian airstrike on an Ahrar al-Sham base, in the village of Tramla, killed Abd Allah Dughaim, known as "Abu Hazim", a commander-in-chief of the Jund al-Sunnah brigade.

Russian assaults against the rebel-held Idlib province have escalated over the past week despite the ceasefire accords.

Last week at least 26 people were killed after suspected Russian airstrikes flattened a number of multi-storey buildings in north-west Idlib city.

Saturday's Russian airstrike closely follows a suspected US drone strike against another Ahrar al-Sham commander in Idlib last Sunday.

The targeting of Ahrar al-Sham commanders by both US and Russian forces raises questions over the coming together of positions between the Russians and the new US administration that will see a wider array of Syrian opposition groups targeted under the pretext of fighting IS militants and groups like the formely al-Qaeda allied Jabhat Fateh al-Sham.

Meanwhile, the Free Syrian Army succeeded in encircling IS-held al-Bab on Sunday.

A source told °®Âþµº that Syrian opposition forces, backed by Turkish forces, besieged al-Bab on all fronts, "imposing significant losses in the ranks of the militant group".

One Turkish soldier was killed and three others wounded in the fighting.

Turkish forces have backed the campaign inside Syria seeking to clean from their border zone the presence of IS militants as well as halting the advance of Kurdish militias nearby.

"All major supply routes into al-Bab have been blocked off, "the source added.

Syrian rebels as part of the Euphrates Shield Campaign have pushed to drive IS militants from al-Bab and entered the town on Saturday.