One month before peace talks are scheduled to begin between the Syrian government and opposition rebel groups, a Syrian airstrike killed a top rebel commander in the outskirts Damascus, according to activist groups and the Syrian government.

Zahran Allouch, the founder of the Saudi-backed Army of Islam rebel faction, was killed Friday by an airstrike on a meeting of rebel commanders near the Damascus suburb of Otaya, according to the Britain-based Syrian Observatory for Human Rights.

The Observatory said it was unclear in the immediate aftermath whether the airstrike was launched by Syrian or Russian warplanes. But the Syrian military later claimed responsibility in a statement published by the state-run SANA news agency.

The report said that the strike was carried out after a series of aerial reconnaissance operations against groupings of "terrorist" organizations and their headquarters in the rebel-held Damascus suburb of Eastern Ghouta. In addition to Allouch, it said the airstrike killed "a large number of commanders of Ahrar al-Sham and Faylaq al-Rahman," other rebel factions who are often allied with Alloush's Army of Islam.

Alloush's death is a blow to insurgents fighting to topple President Bashar Assad and a boost to government forces who have been bolstered by the Russian military intervention in Syria in the past few months. Syrian government forces have been on the offensive in several parts of the country since Russia began its military campaign in late September to shore up Assad's forces.

It was not immediately clear how Allouch's killing would affect his group, which is entrenched in the eastern suburbs of Damascus. 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 last year.

The Army of Islam took part earlier this month in an opposition meeting held in Saudi Arabia to agree on an opposition delegation that would negotiate with government representatives in peace talks planned for late January in Geneva. The Syrian government has always said it will not negotiate with terrorists and considers the Army of Islam to be terrorists.

Allouch, who was in his mid-40s, was widely known to be supported by Saudi Arabia and Turkey. He was one of the most powerful rebel commanders with thousands of fighters controlling large parts of the eastern Damascus suburbs of Eastern Ghouta and Douma. In addition to fighting government forces, the Army of Islam faction fought pitched battles against its rival, the Islamic State group near Damascus.

A former prisoner who was released in a general amnesty after the uprising against Assad began in March 2011, Allouch joined the armed opposition and formed the Army of Islam — which became one of the most organized rebel factions in Syria.

Critics accused him of sectarian politics and brutal tactics similar to that of the Islamic State group.

He is blamed by other opposition groups for the December 2013 kidnapping of four prominent activists including human rights activist and lawyer Razan Zaytouni. He denies holding them although they were kidnapped from an area under Army of Islam control.

Allouch regularly made sectarian comments against members of Assad's Alawite sect as well as Shiite Muslims, although when he spoke to western media earlier this year he said all sects should be part of the rule in future Syria.

Earlier this year, after government airstrikes on the suburbs of Damascus killed dozens, Allouch placed some Alawites that his group was holding in cages in public areas and markets, using them as human shields to try to prevent further airstrikes. Men and women were put in large metal cages on pick-up trucks that drove around Damascus suburbs.

Most of the Syrian militants, like Allouch, are Sunni Muslims and see themselves as oppressed by Alawites.

His group was behind the shelling of Damascus in recent months with mortar shells, including an attack that targeted the Russian Embassy. Such attacks have killed and wounded scores of civilians.

Meanwhile, Pope Francis issued a Christmas Day prayer that the recent United Nations-backed peace process would end suffering in war-torn Syria. He also praised countries that have taken in refugees.

Also on Friday, the Stockholm-based Assyrian Human Rights Network announces that Islamic State militants released 25 more Assyrian Christian hostages that had been held captive in Syria for 10 months. The group said the release brings to 148 the number of Assyrian hostages that have been released so far.

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Associated Press writer Zeina Karam contributed to this report.