Rebels fighting in Syria have given rivals from an al-Qaeda affiliate 24 hours to surrender, according to activists in the country.

Saturday's ultimatum comes amid days of deadly infighting between the Islamic State of Iraq and the Levant (ISIL) and other opponents of Syrian President Bashar al-Assad in two northern provinces, Idlib and Aleppo.

The Britain-based Syrian Observatory for Human Rights said rebels from an alliance of Islamist groups attacked positions of the fighters 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 resistence to the al-Qaeda-linked group's grip on several areas of the country.

Al Jazeera's Zeina Khodr, reporting from Beirut in Lebanon, said the developments could signal a turning point in the war.

"This is the most serious violence between armed opposition and ISIL," our correspondent said.

Sixteen ISIL fighters 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, an activist on the ground, described it as "the start of the revolution against ISIL", according to AFP news agency.

ISIL and Western-backed rebel forces are all fighting to overthrow Assad's regime, but tensions between different groups have been rife in recent months.

Several opposition factions, including a number of fighters united under the name "Army of Mujahedeen", were involved in Friday's fighting, according to the Observatory and local activists.

Both the Islamic Front and the Syrian Revolutionaries Front, two key groups made up of tens of thousands of opposition fighters, also condemned ISIL on Friday.

Anti-ISIL protests

The fighting comes days after ISIL reportedly tortured and murdered a leading opposition figure, doctor Hussein al-Suleiman, known as Abu Rayyan.

His death was the latest in a string of beatings, kidnappings and killings attributed to the group, and prompted protesters to take to the streets under the slogan, "Friday of the martyr Abu Rayyan".

Amateur video shot in Aleppo on Friday reportedly showed protesters chanting: "Free Syrian Army forever! Crush ISIL and Assad!"

Abu Leyla, an Idlib-based activist, told AFP via the Internet: "I'd say about 90 percent of people in the opposition areas are against ISIL".

"They use violence and abuses to crush dissent. They are only Islamic in name. All they want is power," he said.

More than 130,000 people have been killed since the war in Syria broke out in 2011.