Zahran Alloush, who spoke at a wedding in July, led the Army of Islam insurgents in Syria.
"The Russian airstrike hit a building near Misraba district of Eastern Ghouta, where major rebel commanders of al-Islam Army were holding a meeting, including the prominent leader of the group Zahran Alloush", media activist and rebel spokesman Jihad Mustafa told ARA News in Damascus. Speaking to reporters after his talks with Russian foreign minister, Sergei Lavrov, in Moscow, the Qatari foreign minister, Khalid bin Mohammad al-Attiyah, accused Assad of supporting terrorist groups.
Allouch was a controversial figure in the war and an authoritative rebel leader who commanded thousands of fighters on the doorstep of Damascus, the seat of Assad's power. On Saturday, thousands of Islamic State fighters and civilians had planned to evacuate the Palestinian refugee camp of Yarmouk.
However, the Syrian Observatory for Human Rights (SOHR) said the withdrawal had been paused for "logistical" reasons - principally, that the road to Isis-controlled Raqqa had not been secured. Several rebel group commanders have been killed in the past - including most of the command of the ultraconservative Ahrar al-Sham group in a mysterious bombing in northern Syria a year ago.
It could also thwart potential peace talks between the regime and the opposition, which the United Nations announced could begin in less than one month.
Other rebel groups like the Tawhid Brigade of northern Syria did not after its commander Abdul Qader Saleh was killed in an air strike back in November 2013; it rapidly imploded and splintered.
The rebels' capitulation was forced by a government siege over several years that squeezed the flow of food and humanitarian aid, starving many people to death in what rights group Amnesty International has described as a war crime.
It agreed to future negotiations with Syria's regime, a controversial move that drew condemnation from hardline and jihadist groups like Al-Qaeda's Syria affiliate.
The killing of Alloush has dealt a strong blow to the rebel groups operating in the surrounding of Damascus, amid reports that the Islam Army has named a man called Abu Homam al-Bwaidani as the successor of Alloush.
Alloush, head of the Army of Islam group, was killed in an airstrike that targeted the group's headquarters during a meeting.
He reflected the difficulties in identifying moderate rebels from extremists and other militants in Syria. It was established early during the Syria War and now boasts of 15,000-20,000 fighters, according to western intelligence sources.
Alloush and his faction had not been universally accepted in the Syrian opposition - they are widely blamed for the disappearance of four secular opposition activists from the Damascus suburb of Douma.
Government supporters welcomed Alloush's death, blaming him for periodic shelling of civilian areas in Damascus.
Syria's state television channel also reported the death by air strike, but did not say who had carried out the raid.
Alloush's forces receive backing from Saudi Arabia, which has supported Syrian rebels but has thrown its weight behind the renewed diplomatic push to end the conflict.
No one immediately claimed responsibility for the slaying. It doesn't just mark a big threat to the rebel control over the eastern suburbs of Damascus but "disarray among the rebel forces could also consolidate Syrian President Bashar al-Assad's control over the rest of the area", notes Reuters.