Rebels killed and captured dozens of jihadists loyal to an Al-Qaeda affiliate in two days of unprecedented fighting in the Syrian provinces of Aleppo and Idlib, a monitor said Saturday.
Three powerful rebel alliances joined forces in the fighting against the Islamic State of Iraq and the Levant, a move the main opposition National Coalition said it "fully supports".
And in a new sign of the war spreading, ISIL claimed responsibility for a suicide bombing targeting the southern Beirut stronghold of Shiite Hezbollah, which has sent fighters into Syria to back forces loyal to President Bashar al-Assad.
"At least 36 members and supporters of the Islamic State of Iraq and the Levant have been killed since Friday in Idlib and more than 100 have been captured by rebels" in Aleppo and Idlib, said the Syrian Observatory for Human Rights.
The reports come a second day into major clashes in opposition areas of the northern and northwestern provinces between ISIL and rebel alliances which include the massive Islamic Front and the Syrian Revolutionaries Front.
Jihadists who flocked to Syria to join the rebels in their fight against Assad's regime were at first welcomed by the armed opposition.
But relations grew bitter after ISIL fought other opposition groups for control and committed systematic abuses against activists and rival rebels.
The Assad regime has systematically branded rebels and peaceful activists alike as "terrorists" since the start of an uprising against his regime in March 2011.
But anti-Assad activists have in the past 48 hours described the escalation against ISIL as a new "revolution" in Syria.
Since Friday, rival rebels have "seized checkpoints, bases and weapons from ISIL" in Aleppo and Idlib provinces, said Observatory director Rami Abdel Rahman.
ISIL was reported to have kidnapped, beaten and executed dozens of rival rebels and activists since it appeared in Syria before last summer, establishing a reign of terror in areas where it operates.
It has forced young schoolgirls in some areas to wear the veil should they wish to go to school, and lashed and executed people -- including children -- on accusations of heresy.
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The escalation came as the nascent Army of Mujahideen, a new rebel alliance, declared all-out war on ISIL.
"We, the Army of the Mujahideen, pledge to defend ourselves and our honour, wealth and lands, and to fight ISIL, which has violated the rule of God, until it announces its dissolution," the new alliance of eight groups said in a statement posted online.
The alliance demanded ISIL fighters either join the ranks of other rebel groups "or hand over their weapons and leave Syria".
The National Coalition said it supports the rebels' efforts, while calling on "the international community to recognise the importance of supporting revolutionary forces as partners in the fight" against both Al-Qaeda and Assad.
"The Syrian Opposition Coalition fully supports ongoing efforts by Free Syrian Army elements to liberate towns and neighbourhoods from the authoritarian oppression" of ISIL, its presidency said.
'Syria, Iraq conflicts melting into one'
In a fresh sign that the Syria conflict is spreading through the region, ISIL claimed credit for a deadly bombing that killed four people Thursday in the southern Beirut stronghold of Hezbollah.
ISIL said in a statement posted online that it had penetrated the "security system of the Party of Satan (Hezbollah)... and crush its strongholds... in a first small payment from the heavy account that is awaiting those wicked criminals".
Thursday's suicide car bomb in the Haret Hreik district of the Lebanese capital also wounded 77.
It was the latest strike against the powerful party, whose fighters are aiding Assad in the civil war that pits his troops against a Sunni-led rebellion.
Earlier Saturday, Lebanon's army had confirmed a young man from northern Lebanon was the bomber who blew himself up.
The reports come as an Iraqi security official told AFP Fallujah fell to ISIL militants.
According to Aron Lund, editor of Syria in Crisis website run by the Carnegie Endowment, "the two conflicts in Iraq and Syria are melting into one. The more conflicts you pull into the Syria war the harder it will be to stop it".