Islamists fighting to topple President Bashar al-Assad forged Syria's largest rebel alliance Friday and pledged to work towards an Islamic state, as rebels seized a town on the Lebanese border.
The merger of the six groups comes after repeated calls for unity from opposition fighters and their foreign backers, following advances by regime forces around Syria's main cities of Damascus and Aleppo.
"The 'Islamic Front' is an independent military and social force that is aimed at bringing down Assad's regime in Syria and at replacing it with a just Islamic state," a statement said.
Earlier, a rebel spokesman, Abu Firas, declared "the complete merger of the major military factions fighting in Syria."
Speaking to AFP via the Internet from the northern province of Aleppo, Abu Firas said the Front, bringing together tens of thousands of rebels, would have "one policy and one military command."
Among those joining are Aleppo's biggest fighting force, Liwa al-Tawhid, the Salafist Ahrar al-Sham and the Army of Islam, which is concentrated around Damascus.
Creation of the joint force followed criticism from opposition sources and experts partly attributing recent rebel battleground losses to disunity.
But it threatens to undermine the foreign-based leadership of the mainstream Free Syrian Army, which has gradually lost credibility over its inability to secure weapons demanded by fighters.
Aron Lund, an expert on the 32-month conflict, described as significant the amalgamation of mainstream and hardline Islamists, excluding any Al-Qaeda factions.
"It's something that could be very important if it holds up," Lund told AFP.
The Front's formation was a response to both regime advances and the "aggressive posture" of jihadists against other rebels, he said, adding: "I assume there's a good deal of foreign involvement as well."
"There's been a lot of talk about how Saudi (Arabia) and the Gulf have been pushing to unify the rebels."
The move came days after the death of Liwa al-Tawhid's charismatic military chief Abdel Qader Saleh, who had reportedly called for unity.
'Bad news for Assad and ISIL'
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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."
"It has been decided that all the factions' military, media, humanitarian and administrative offices will merge over a transitional period of three months."
The news came as anti-Assad protesters in Syria took to the streets for weekly demonstrations, under the rallying cry "The blood of the martyr (Saleh) unites us."
Saleh died from his wounds Monday, after an air strike on a building in Aleppo where he and other faction leaders had been meeting.
Activists welcomed the merger as "bad news" both for Assad and the jihadist Islamic State of Iraq and the Levant, which has fought against some rebel brigades in opposition-held areas, including Islamists.
"The news will terrorise the regime and ISIL at the same time," said one activist group.
In the latest fighting, rebels, including jihadists, seized Deir Attiya on the Lebanese border, said the Syrian Observatory for Human Rights.
The majority Christian town in the Qalamoun area north of Damascus is on the strategic route linking the capital to Homs in central Syria.
It was seized by ISIL and Al-Nusra Front, as well as other Islamists, said the Observatory.
Three days earlier, the army took nearby Qara, which had been under opposition control for months.
Regime warplanes staged 16 air strikes on Qalamoun Friday, the Observatory said.
On the diplomatic front, UN-Arab League envoy Lakhdar Brahimi held talks Friday in Geneva with Iranian Foreign Minister Mohammad Jarad Zarif, whose country is a key Damascus ally.
The half-hour meeting was part of "preparations for Geneva II," Brahimi's spokeswoman said, referring to a long-delayed peace conference on Syria.
Meanwhile, UN refugee chief Antonio Guterres urged the European Union to step up efforts to help countries like Bulgaria cope with an influx of Syrian refugees, after visiting a refugee camp in Sofia.
More than 120,000 people have died in Syria's war, which erupted after Assad's regime launched a crackdown on pro-democracy protests, sparking a brutal insurgency.