Syrian opposition groups mourned the death of Zahran Allouch, the head of Jaish al-Islam (Army of Islam), which operates in Ghouta and Douma near Damascus and also fights Daesh near the Qalamoun Mountains. The opposition groups claimed that he was killed in a Russian airstrike on Friday although the Syrian regime claimed the responsibility. Witnesses said on Twitter he was killed along with in an airstrike that targeted the group's headquarters during a meeting on Friday. He was instantly killed along with a number of commanders from the ultraconservative Ahrar al-Sham and the Faylaq al-Rahman 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. Allouch's killing a month before peace talks are scheduled to begin between the Syrian government and opposition rebel groups is a blow to insurgents fighting to topple Assad and a boost to government forces who have been bolstered by the Russian military intervention in Syria.The Army of Islam took part earlier this month in an opposition meeting held in Saudi Arabia during which it agreed to take part in political talks seeking to end the five-year-old conflict scheduled for late January in Geneva. The Syrian government describes the group as "terrorists" and has said it will not negotiate with such factions. The U.N. envoy for Syria, Staffan de Mistura, said Saturday that he has set a Jan. 25 target date for the talks in Geneva and said developments on the ground "should not be allowed to derail it." Anas al-Abdeh, a senior member of the main Western- backed opposition group, the Syrian National Coalition, said the assassination "makes a mockery of all talk of a political settlement" and undermines the "negotiations before they begin." Several opposition groups also mourned Allouch's death and accused the government and its allies of trying to eliminate rival groups ahead of the talks. Several rebel leaders have been killed since Russia's aerial campaign started on Sept. 30 in support of ally Assad, although Moscow has insisted that it is concentrating its attacks on Daesh."Rebel groups should realize they are facing a war of extermination by (Russian President Vladimir) Putin's regime," said Labib Nahhas, a senior member of the militant rebel Ahrar al-Sham group. Abu Hassan al-Muhajer, another senior member of Ahrar al-Sham, wrote on Twitter that the "next stage will witness the liquidation of those leaders who began the uprising" against Assad. Other insurgents, including the al-Qaida branch in Syria, the Nusra Front, also lamented the killing.The Army of Islam swiftly appointed Essam al-Buwaydhani, a field commander known as Abu Hammam, as Allouch's successor, and posted a video on the Internet late Friday saying Allouch's killing "will only increase our fight" against Assad's government and Daesh. However, Aron Lund, a Syria expert, said the death of Allouch, who led the Army of Islam since it was founded around four years ago, could amount to "a decapitation strike" for the group. "Add to that the fact that the Islam Army's dominance has created so much resentment among other factions over the years, and the situation seems very unstable," Lund wrote in an analysis for the popular Syria Comment blog.Government supporters along with Daesh members celebrated his death, blaming his group for regularly shelling residential areas in Damascus. The state-run SANA news agency said nine civilians were killed and 18 were wounded later Saturday by mortar shells fired by the Army of Islam on districts in and around Damascus. After nightfall, the Army of Islam claimed on its Twitter account that it has attacked a checkpoint near Damascus, killing 28 Syrian troops. The group also posted a video message by its new leader, showing him with a long beard and in a uniform, pledging to continue on the same path as Allouch. Alloush's death leaves so much up in the air, Lund wrote, especially in the Damascus suburbs of Ghouta. "If it leads to instability and infighting among the rebels, or weakens command and control in the Ghouta, we could start to see a shift in the balance of power in the Syrian capital over the coming months," he said.Jaish al-Islam is known for its successful operations against the regime and Hezbollah near Damascus. In an article published in Foreign Policy on Oct. 1, 2013, the group was described as the winning side after unifying more than 50 small groups under its umbrella. The article claims that the U.S. and Gulf countries, mainly Saudi Arabia, have provided aid to the group based on a report published by The New York Times on March 24, 2013. The article also claims that the U.S. was hoping that the group would stop the rise of extremist groups in Syria. "The formation of the Army of Islam in the capital's eastern fringe under Zahran Alloush, the leader of the group Liwa al-Islam, strengthens Salafist militants owing allegiance to Riyadh against Daesh," a Reuters report said on Oct. 1, 2013. "While hoping to avoid outright confrontation with fellow militants, the Saudis had been gauging the willingness of local Salafist fighters in joining Saudi-backed formations, including a proposed Syrian National Army," the same report said. In sum, Jaish al-Islam was supposed to both stop Daesh and al-Qaida-affiliated groups and disturb the regime. Neither Daesh nor al-Nusra Front is effective in the south.