Saturday's ultimatum comes amid days of deadly infighting between the terrorist group of the Islamic State of Iraq and the Levant (ISIL) and other opponents of the Syrian government in two northern provinces of Idlib and Aleppo.

A report said militants from an alliance of groups attacked positions of the terrorists from ISIL, killing and capturing dozens of people on Saturday.

The ISIL has been blamed for brutal killings in areas under their control, turning many local residents against them and leading to a growing resistance to the al-Qaeda-linked group's grip on several areas of the country.

"This is the most serious violence between armed opposition and ISIL," an al-Jazeera reporter said.

Sixteen ISIL terrorists were reportedly killed in the fighting in Aleppo and nearby Idlib on Friday, while at least 42 other ISIL fighters were wounded in Idlib alone.

Meanwhile, protesters in opposition-held parts of Syria chanted slogans condemning the al-Qaeda affiliate.

Ammar, a militant on the ground, described it as "the start of the revolution against ISIL", according to AFP news agency.

ISIL and foreign-backed militants are all fighting against the Syrian government, but tensions between different groups have been rife in recent months.

Several opposition factions, including a number of militants united under the name of "Army of Mujahideen", were involved in Friday's fighting, according to the Syrian Observatory for Human Rights and local opposition.

Foreign-backed militants of the so-called Islamic Front and the Syrian Revolutionaries Front have also condemned ISIL terrorists over their atrocities in the war-torn country.

Syria has been gripped by deadly unrest since 2011, when foreign-backed terrorists hijacked pro-reform protests of the Syrian nation.

According to reports, Western powers and their regional allies - especially Qatar, Saudi Arabia, and Turkey - support the militants operating inside the country.

NTJ/HH