Alloush, 44, was the commander of the Jaish al-Islam (Army of Islam) movement, the predominant opposition faction in the Eastern Ghouta rebel bastion east of Damascus.

Hours after Alloush was killed, leading members of Jaish al-Islam elected Abu Himam al-Buwaydani as a replacement, the Syrian Observatory for Human Rights told AFP.

Buwaydani is a 40-year-old businessman and fighter from Douma who hails from a family with strong ties to the Muslim Brotherhood, Observatory director Rami Abdel Rahman said.

Alloush and five other commanders were killed "in an air strike that targeted one of their meetings in Eastern Ghouta" on Friday, the Observatory said, adding that it was unclear whether the regime or Russia had been behind the raid.

A senior member of Jaish al-Islam confirmed Alloush`s death to AFP, saying three planes targeted a "secret meeting" of commanders.

The Syrian government and its media regularly refers to Jaish al-Islam as "terrorists," and state television did so again in the news alert announcing Alloush`s death, saying that Syria`s army command had conducted the "special operation" that killed Alloush as part of its "national mission."

A Syrian security source told AFP "dozens" of rebel fighters were killed in the raids, carried out by Syria`s air force with newly-provided Russian missiles.

The jets launched two rounds of strikes on the meeting with four missiles each, the source said. At least 12 Jaish al-Islam members and seven from the Islamist Ahrar al-Sham group were killed.

Jaish al-Islam is the most prominent rebel faction in the Eastern Ghouta region, an opposition bastion east of the capital frequently bombarded by regime forces.

Backed by Riyadh, it recently took part in landmark opposition talks in Saudi Arabia.

It was known to have extremist views and to have supported the establishment of an Islamic state before recently moving towards a more moderate position.

Born to Salafist sheikh

Zahran Alloush was born in 1971 in Douma, one of the largest towns in Eastern Ghouta. His father was a prominent Salafist preacher who now resides in Riyadh.

Following in his father`s footsteps, Alloush pursued religious studies in both Syria and Saudi Arabia.

Alloush was arrested in 2009 and was released in June 2011 in a general amnesty, just three months after Syria`s uprising against President Bashar al-Assad erupted.

He took up arms, and in 2013 united a number of rebel groups under the banner of Jaish al-Islam.

With close-cropped hair and a dark beard, Alloush was typically dressed in military-style fatigues.

Jaish al-Islam rose to prominence in Eastern Ghouta and has remained firmly opposed to both Assad and to the Islamic State jihadist group.

But the group has also generated severe criticism by human rights groups for rights abuses.

In July, they drew condemnation for executing 18 alleged members of the Islamic State group in a video mimicking IS`s own gruesome productions.

And in November, Jaish al-Islam used dozens of captives in metal cages as "human shields" in an attempt to "prevent regime bombardment" of Eastern Ghouta, according to the Observatory.

The news of the air strikes killing Alloush spread like wildfire among activists and rebel groups online.

"May God accept Commander Zahran Alloush among the martyrs... and may the factions of Ghouta join forces to bridge the gaps and complete the mission," wrote Khaled Khoja, head of the opposition National Coalition, on Twitter.

`Significant opposition loss`

Analysts expect Alloush`s death to have profound ripple effects on Syria`s fragmented rebel movement as well as budding peace talks.

His death "stands as one of the most significant opposition losses" of Syria`s nearly five-year uprising, analyst Charles Lister said on Twitter.

"In a way, Zahran Alloush has been the rare successful centraliser in the Syrian rebel movement," said Aron Lund, editor of the Carnegie Endowment`s Syria in Crisis website.

But with Alloush gone, that cohesion could "unravel," Lund added.

His absence could also affect a fragile peace process aimed at negotiating a political solution to Syria`s war, in which more than 250,000 people have died.

Jaish al-Islam was one of the leading rebel groups invited to talks in Riyadh earlier this month in a bid to hash out a common platform.

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.

"Those negotiations needed hardliners like Zahran Alloush to be involved for their credibility," Lund said.

But Alloush`s death "could affect the peace process, both by destabilising Jaish al-Islam and by weakening it," he added.

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