Fighters from several Syrian rebel factions have engaged in fierce battles against the Islamic State of Iraq and the Levant (ISIL), in what activists say is growing resistance to the al-Qaeda-linked group's grip on several areas of the country.
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.
Al Jazeera's Zeina Khodr, reporting from Beirut in Lebanon, said the developments could signal a turning point in the war.
"We are seeing clashes in more than one area, which could signal the coming of a major clash," our correspondent said.
ISIL and Western-backed rebel forces are all fighting to overthrow the regime of President Bashar al-Assad, 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 Britain-based Syrian Observatory for Human Rights 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.
"We call on ISIL to withdraw immediately from Atareb... and remind them that those who freed Atareb [from the Syrian government] are those you are fighting today," the Islamic Front said.
In Idlib, 20 civilians were injured in the fighting, while a media activist was killed in Aleppo, AFP reported.
The fighting comes two 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!"
Another video shot on Friday from the opposition-held town of Kafr Takharim, in Idlib governorate, showed protesters running through the street as gunfire rang out. The Observatory and opposition activists said ISIL fired on the protesters.
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.
Opposition-held areas of northern and eastern Syria have seen a wave of kidnappings over the past six months that has targeted journalists, aid workers and activists. Al-Qaeda-linked groups are suspected of being behind many of the abductions
More than 130,000 people have been killed since the war in Syria broke out in 2011.