Six key Islamist groups fighting President Bashar al-Assad's regime in Syria joined forces on Friday, a spokesman for Aleppo's biggest rebel force said.
"Thank God, the complete merger of the major military factions fighting in Syria has been announced," Liwa al-Tawhid spokesman Abu Firas said in a posting on Facebook.
The creation of the joint force -- which has been dubbed the Islamic Front -- came after major regime advances on key battlegrounds around Damascus and Aleppo in northern Syria.
Opposition sources and experts have attributed the regime's successes in part to rebel disunity.
The groups merged days after the death of Liwa al-Tawhid's charismatic military chief Abdel Qader Saleh, who had reportedly made calls for such a rebel alliance.
According to Abu Firas, the groups merging their troops were Liwa al-Tawhid, Ahrar al-Sham, the Army of Islam, Suqour al-Sham, Liwa al-Haq and the Ansar al-Sham battalions.
All the groups are Islamist and merged "under the banner of 'There is no god but God, and Mohammed is his prophet'", said Abu Firas, citing the Islamic profession of faith.
Speaking to AFP over the Internet, Abu Firas said "the doors are open to all the military factions, and a committee is working to study the entrance of all groups that also want to join" the merger.
"It has been decided that all the factions' military, media, humanitarian and administrative offices will merge over a transitional period of three months," he added.
A spokesman for another rebel faction that had joined the merger confirmed the report, and said an official statement would be released "shortly".
News of the merger came as anti-Assad protesters in Syria took to the streets for their weekly demonstrations, this time under the rallying cry "The blood of the martyr (Abdel Qader Saleh) unites us".
Saleh died from his wounds Monday, after an air strike hit the building in Aleppo where he and other faction leaders were meeting days earlier.
Activists on the ground welcomed news of the merger as "bad news" both for Assad's regime and for the jihadist Islamic State of Iraq and the Levant, which has fought against some rebel brigades in opposition-held areas.
"The news will terrorise the regime and ISIL at the same time," said another activist group.
Also welcoming the merger, Aleppo-based activist Abu Omar told AFP it "will help bring control on all the liberated (opposition-held) areas in Syria, and prevent the emergence of divided statelets".
As well as daily violence, rebel areas are racked with insecurity and criminality that flourish with the lack of a centralised authority.