BEIRUT: Syrian rebels in opposition-held areas were engaged in fierce battles with Al-Qaeda-linked elements Friday in what activists say is growing resistance to the jihadists' brutal grip in many places.
Meanwhile, Doctors Without Borders (MSF) said five of its staffers were taken from a house in the north of the country by an unknown group.
Protesters turned out in rebel areas chanting the strongest slogans yet against the Islamic State of Iraq and the Levant (ISIL), as tensions soar between the Al-Qaeda affiliate and the opposition.
Ammar, an activist on the ground, described Friday's protests as "the start of the revolution against ISIL."
The Syrian Observatory for Human Rights reported fierce fighting between rebels and ISIL in several flashpoints in Aleppo city and province.
In Aleppo and nearby Idlib, 16 pro-Qaeda fighters were reported killed.
The Britain-based Observatory said a number of battalions united under the name "Army of the Mujahedeen" fought fierce battles against ISIL around Atareb in Aleppo province.
And activists said a number of local rebel groups, including more moderate Islamists, were fighting ISIL.
In Aleppo city, activist Nazeer al-Khatib reported clashes in several neighbourhoods.
The fighting comes two days after ISIL reportedly tortured and murdered Doctor Hussein al-Sleiman, known as Abu Rayyan.
His death was the latest in a string of beatings, kidnappings and killings and has enraged rebels and activists alike.
It prompted protesters to take to the streets of several towns under the slogan: "Friday of the martyr Abu Rayyan."
Amateur video shot in Aleppo city showed protesters chanting: "Free Syrian Army forever! Crush ISIL and Assad!"
A second video from opposition-held Kafr Takharim showed protesters running through the street as gunfire rang out in the background.
The Observatory and activists accused ISIL of firing on the protesters, who were chanting anti-regime slogans as they have every week since the outbreak of an uprising that has killed more than 130,000 people.
ISIL uses 'violence, abuse to crush dissent:' activist
Both the Islamic Front and the Syrian Revolutionaries Front, two key alliances formed last year that group tens of thousands of fighters, condemned ISIL.
"We call on ISIL to withdraw immediately from Atareb... and remind them that those who freed Atareb (from Assad's regime) are those you are fighting today," said the Islamic Front.
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 Islamist in name. All they want is power," he added.
Syria's revolt began as a peaceful Arab Spring-inspired movement demanding the end of the Assad family's four-decade rule that was met with a brutal crackdown by the regime.
That sparked an armed uprising, and foreign jihadists soon flocked to Syria to join the rebels.
The jihadists were welcomed at first, but "their abuses have made it impossible for them to stay here. We want freedom, not ISIL," said Abu Leyla.
MSF staffers 'taken'
Meanwhile, five MSF staffers were taken from a house in northern Syria by an unknown group, "apparently for questioning," said Samantha Maurin, a spokeswoman for the international medical NGO.
It was unclear who had taken them, and MSF declined to release details about them or where they had been.
MSF has six hospitals and four health centres in northern Syria and provides health support from neighbouring countries to within Syria as well as to Syrian refugees.
ISIL has been accused of targeting both foreign and Syria journalists as well as aid workers and activists for kidnapping.
In a separate development, Danish and Norwegian vessels left Cyprus and headed towards Syria to escort a delayed shipment of chemical weapons for destruction, said Norwegian army spokesman Lars Magne Hovtun.
They will "set a course toward a holding area in international water outside Syria, so we are most ready to enter the port of Latakia when the order arrives," Hovtun added.
The ships are to be joined by Chinese and Russian vessels inside Syrian waters.
The removal had been scheduled to take place before December 31, but the deadline passed and a new one has not yet been set.
The year-end deadline was the first major milestone under a UN Security Council-backed deal arranged by Russia and the United States that aims to eliminate all of Syria's chemical arms by mid-2014.