BEIRUT - The assassination of a top Syrian rebel commander who led one of the most powerful groups battling President Bashar al-Assad's forces has dealt a significant setback to the opposition that could reshuffle the lineup of key players on the ground ahead of the planned peace talks in Geneva next month.

On Saturday, the Army of Islam and allied militant groups in Syria mourned the killing of Zahran Alloush, while government supporters and the Islamic State group cheered his death - a reflection of his role in fighting both sides in the Syrian civil war.

Allouch was killed in airstrikes that targeted the group's headquarters during a meeting on Friday. He died along with a number of senior commanders of his Army of Islam group and those of the ultraconservative Ahrar al-Sham and Faylaq al-Rahman groups.

The Syrian army claimed responsibility for the airstrike that killed Allouch, although many among the opposition blamed Russia, which has been bombing IS targets and other insurgent groups since late September.

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. His death may have contributed - at least partially - to a delay in an agreed-on pullout of thousands of militants and their families from neighborhoods on the southern edge of Damascus.

The pullout, supposed to start Saturday, was to involve mainly militants from IS who earlier this year overran the Yarmouk area, which is home to a Palestinian refugee camp and has been hotly contested and fought-over in the war, and two adjacent neighborhoods.

A Palestinian official in Damascus, Anwar Abdulhadi, told the Associated Press that the withdrawal is being delayed for "logistical reasons." But Lebanon's Hezbollah-run TV station Al Manar said that Allouch was a key figure in arranging the rare deal, and that his assassination has delayed its implementation. The report could not be immediately confirmed by the AP.

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.

The Army of Islam took part this month in an opposition meeting held in Saudi Arabia during which it agreed to participate in political talks, scheduled for late January in Geneva, seeking to end the five-year-old conflict. The Syrian government describes it as a terrorist group 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 talks "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. A number of rebel leaders have been killed since Russia's aerial campaign started Sept. 30 in support of ally Assad, although Moscow has insisted that it is concentrating its attacks on Islamic State, also called IS and ISIS.

"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 ISIS.

However, Aron Lund, a Syria expert, said the death of Allouch, who led the Army of Islam since it was founded, could amount to "a decapitation strike" for the group.

A former Islamist prisoner who was released in a general amnesty after the uprising against Assad began in March 2011, Allouch, is in his mid-40s when he died, joined the armed opposition and formed the Army of Islam - which soon became one of the most organized rebel factions in Syria, based in the Damascus suburb known as Eastern Ghouta.

He was widely known to be supported by Saudi Arabia and Turkey but also fought pitched battles against rival ISIS, with many crediting his group for keeping IS from making further advances toward the Syrian capital. But critics accused him of sectarian politics and brutal tactics similar to those of ISIS.