"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 Idlib and Aleppo, said the Syrian Observatory for Human Rights.
The reports come a second day into 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 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, as well as ordinary citizens.
Assad's regime has branded both rebels and peaceful activists as "terrorists" since the start of an uprising against him in March 2011.
But in the past 48 hours, anti-Assad activists have described the escalation against ISIL as a new "revolution" in Syria, given ISIL's increasing number of kidnappings and beheadings and other abuses.
Rival rebels have "seized checkpoints, bases and weapons from ISIL" in Aleppo and Idlib, said the Observatory.
ISIL was reported to have kidnapped, beaten and executed dozens of rival rebels and activists since it appeared in Syria, establishing a reign of terror in areas it controls.
It has forced girls to wear the veil to school, and lashed and executed people -- including children -- on accusations of heresy.
The escalation comes as the nascent Army of Mujahedeen, a new rebel alliance, declared all-out war on ISIL.
"We, the Army of the Mujahedeen, 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," said the new alliance of eight groups.
It demanded ISIL fighters either join the ranks of other rebel groups "or hand over their weapons and leave Syria".
The opposition 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 Coalition presidency said it "fully supports ongoing efforts by Free Syrian Army elements to liberate towns and neighbourhoods from the authoritarian oppression" of ISIL.
'Syria, Iraq conflicts melting into one'
On Saturday, ISIL gave the forces aligned against it 24 hours to stop their attacks, release their prisoners and remove checkpoints, or it would withdraw from Aleppo, allowing government forces to enter.
"The withdrawal of the Islamic State from any of those points will mean the invasion... by the criminal regime," said an ISIL statement obtained by the SITE Intelligence Group.
The Mujahedeen issued a new appeal for ISIL fighters to "defect and to join your honest brothers who are fighting Assad across Syria".
It came after the Sunni extremist group claimed a deadly bombing that killed four people Thursday in Haret Hreik, a district in the southern Beirut stronghold of Hezbollah.
The attack was the latest against the powerful Shiite party, whose fighters are aiding Assad in the civil war that pits his troops against a Sunni-led rebellion.
The reports come as an Iraqi security official told AFP Fallujah had fallen to ISIL.
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".