World Bulletin / News Desk

At least 20 opposition commanders have died in Syria since early December in a string of mysterious targeted killings, a monitor said on Wednesday.

The commanders have been killed in roadside bombs or shootouts, but no faction has claimed responsibility for their deaths. 

Analysts say they could be part of an assassination campaign carried out by either the regime of President Bashar al-Assad or the ISIL armed group. 

On Tuesday, Abu Rateb al-Homsi, a provincial "emir" in the Ahrar al-Sham opposition group, was killed when unknown attackers fired on his car in the central province of Homs, according to the Syrian Observatory for Human Rights. 

Homsi is the most high-profile of those killed, most of whom come from the ranks of Al-Qaeda's Syria affiliate Al-Nusra Front and groups like Ahrar al-Sham. 

"The assassinations have intensified since the beginning of December, but it's unknown who carried them out," said Observatory head Rami Abdel Rahman. 

Among the dead, seven were leading officers in Al-Nusra, which has a strong presence in northern and southern Syria, as well as outside the capital, Abdel Rahman said.

Hussam Ammura, Al-Nusra's emir in the besieged Palestinian camp of Yarmuk in southern Damascus, was killed on December 22. 

Abu Julaybib, a Jordanian national and Al-Nusra's emir in the southern Daraa province was killed on December 4. 

Ahrar al-Sham has had two senior members killed this month: Homsi, and Abdel Qader Dabaan, a commander in the northwest province of Idlib. 

Another six leading figures with close ties to Ahrar al-Sham and five other commanders have also been assassinated since early December, Abdel Rahman said. 

Ahrar al-Sham and Al-Nusra have formed a strong alliance in northwest Syria, where they have fought both regime forces and ISIL. 

According to Thomas Pierret, a Syria specialist at the University of Edinburgh, "the regime and its allies are by far the main suspects" in the killings. 

"One of the components of the counter-insurgency strategy implemented by Russia since September is the decapitation of the insurgent leadership," he added. 

With help from their Russian ally, intelligence services in Damascus have been able to gather better information on senior opposition figures, Pierret said.

But another possibility, he said, is that ISIL "sleeper cells" carried out the killings. 

"Real or suspected supporters of ISIL are regularly targeted by the opposition in the northernmost parts of Homs province, where Abu Rateb (al-Homsi) was assassinated," according to Pierret. 

Syria's conflict began in March 2011 with anti-government demonstrations, but it has spiralled into a multi-sided war.