BEIRUT: The killing of Syrian rebel chief Zahran Alloush, fiercely opposed to both the regime and Daesh, has eliminated a key bulwark against the militants and could derail U.N.-brokered peace talks, analysts say.

The head of Army of Islam, the foremost rebel group in Damascus province, was killed Friday in an strike claimed by Syria’s government.

The Army of Islam has fought off both government forces and Daesh (ISIS) militants in its Eastern Ghouta bastion, east of the capital.

It even recently agreed to eventual peace talks with the regime of Bashar Assad.

But with Alloush gone, that centralizing force has vanished, says Andrew Tabler, analyst at the Washington Institute for Near East Policy.

“He occupied a space between the extremists and the Free Syrian Army, which was important to hold off the spread of ISIS in the short term and unify the rebels vis-a-vis the regime in the long term,” Tabler says.

Without him, Islamist fighters could lean toward further radicalization and join militant groups, Tabler adds.

Despite Alloush’s ferocious opposition to Daesh, the regime and its media consistently referred to him and to his militia as “terrorists.”

Syria’s army command did so again in the statement claiming responsibility for his death.

Damascus has used the term for all its opponents – including U.S.-backed rebels – since the conflict erupted in 2011.

It has strived to present itself globally as a legitimate government fighting militants across the country.

Alloush’s death is “a major blow to the rebels,” says Karim Bitar of the Institute for International and Strategic Affairs.

“It might temporarily help the regime, as his disappearance reinforces the binary Assad-versus-Daesh dichotomy,” Bitar tells AFP.

It could also thwart potential peace talks between the regime and the opposition, which the U.N. announced could begin in less than one month.

His killing is “a severe blow to the negotiations process. In the past two years, the Saudis have been propping up Alloush and trying to present him as an alternative,” Bitar says. The Army of Islam will need time “to recover from this blow and for the alternative leadership to emerge.”

And given Alloush’s fate, other groups participating in the talks could back out.

“His death shows the severe disincentive of Islamist groups to stay in the negotiations outlined by the U.N.,” Tabler tells AFP.

In a statement released Saturday, U.N. peace envoy Staffan de Mistura said he would aim to convene the landmark meeting between the opposition and the regime on Jan. 25 in Geneva.

He expressed hopes that the “broadest possible spectrum” of opposition representatives would be involved, adding: “Continuing developments on the ground should not be allowed to derail it.”

Backed by Saudi Arabia, the Army of Islam was one of the most influential armed groups invited to broad-based opposition talks in Riyadh earlier this month.

Representatives in the Saudi capital agreed to eventual negotiations with the regime and were set to choose at least part of the opposition delegation for upcoming talks.

The son of a Salafist preacher, he took up arms as part of Syria’s uprising shortly after being released from prison in a general amnesty.

He often expressed his hatred of Shiites and Alawites – the sect of Assad’s clan – but recently began reeling in his sectarian diatribes and attempted to present his group as more moderate or nationalistic.

“Alloush tried to come off as a constructive, responsible centrist, an anti-terrorist ally, and an all-around gentleman,” writes expert Aron Lund on the Syria Comment website.

“You know, the kind you’d like to see in a coalition government.”

With him gone, more hard-line groups could step in to fill the void in Eastern Ghouta.

“It is possible the death will be a significant blow to Jaish al-Islam because the movement has been heavily centered around [Alloush] as an individual,” says Syria analyst Aymenn al-Tamimi, using the Arabic name for the Army of Islam.

Tamimi said other groups in Eastern Ghouta, like hard-line group and Al-Qaeda ally Ahrar al-Sham, will now “move to assert more influence in the political landscape there.”