During one of the Russian air strikes a leader of Jaysh al-Islam, Zahran Alloush has been killed.
The Syrian Observatory for Human Rights cites “leading sources” in Mr Alloush’s rebel group who say that that the commander was killed along with eight of his fellow fighters.
They are believed to have been killed after a meeting in the east of Damascus was targeted by Russian warplanes.
The head of Syrian’s opposition national coalition also said Mr Alloush had been killed and posted a message of condolence to him on Twitter.
Mr Alloush has previously used heavily sectarian language about the Alawite and Shiite ethnic groups and called for them to be “cleansed” from greater Syria.
He was noted as a charismatic leader of the group, and claimed Al Qaeda regional affiliate Al Nusrah front as an ally.
Jaysh al-Islam, also called Army of Islam, controls parts of the Syrian capital Damascus’s suburbs.
The group is in control of Eastern Ghouta, a significant outlying area in the capital region that has been under the control of opposition forces.
The Assad regime’s ground forces has been engaged in an assault on the region to try and claim it back.
Russian air strikes are targeting rebel groups in Syria, including Isis and other groups that have received western support.
The news comes amid reports that Isis and the Syrian government have reached a deal that would see the Islamist militant group withdrew from suburbs in the south of Damascus.