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BEIRUT, Lebanon — Deadly clashes were reported Friday in northern Syria between Sunni Islamist jihadists linked to Al Qaeda and insurgents in other alliances, punctuating a growing schism within the armed Syrian opposition over the power exerted by its religiously radicalized members, many of them from other countries.

Antigovernment activists in the Aleppo area said that fighting had broken out near the Idlib provincial town of Atareb, west of Aleppo, pitting members of the Islamic State of Iraq and Syria, or ISIS, a powerful Qaeda affiliate that includes foreign fighters, against an array of seven homegrown Syrian rebel groups. The rebels call themselves the Mujahedeen Army, and they resent what they see as ISIS's hijacking of their struggle, now nearly three years old, to depose President Bashar al-Assad.

The Mujahedeen Army also issued a statement in Arabic on Facebook essentially announcing that it now considered ISIS an enemy.

"We, the Mujahedeen Army, declare that we will defend ourselves, our honor, properties and land and we declare the fight against the ISIS organization, the unjust to God's law, until it dissolves its formation and its members join other military formations or abandon their arms and leave Syria," the statement read.

Angry demonstrations against ISIS were reported to have erupted elsewhere on Friday in rebel-held areas of northern Syria by civilians fed up with what they see as its dictatorial behavior, which has included arresting, punishing and sometimes executing anti-Assad activists who disagree with the goal of creating a strict monolithic Sunni Islamic state.

Another alliance of anti-ISIS rebels called the Islamic Front issued a statement denouncing what it called a series of crimes committed by ISIS. The alliance said that the crimes included the seizure of property and weapons from other rebel groups, including the Free Syrian Army; assaults and kidnappings of civilians; and, most recently, the torture and execution of Abu Rayan, a prominent activist doctor in the Aleppo area.

The statement exhorted Syrians to "stand firmly and strike with an iron fist all those who are hampering the Syrian revolution path."

The backlash against ISIS in Syria came as the group's fighters in neighboring Iraq were battling an alliance of Iraqi security forces and local tribal leaders in Anbar Province, which borders Syria and has been an incubator of Sunni extremism in both countries.

An antigovernment activist in the Aleppo area reached by Skype said the fighting in Atareb had left an unspecified number of ISIS fighters dead and at least 20 detained, including a Tunisian identified as the local ISIS emir and two Turks. The activist, who identified himself only by his first name, Assaad, for security reasons, said four Islamic Front fighters had been killed.

Another activist reached by Skype in Aleppo, who identified himself as Nazeer al-Khatib, said that clashes between ISIS fighters and other rebels had expanded to "many areas in Aleppo."

The Syrian Observatory for Human Rights, a Britain-based group with a network of observers and informants in Syria that has chronicled the conflict and reports of human rights violations by both sides, said that at least 42 wounded ISIS fighters had been transferred to rural Idlib hospitals, and that least 20 civilians also had been wounded.

In Binnish, another Idlib provincial town, southwest of Aleppo, an activist reached by Skype who identified himself only by his first name, Najid, said that civilians had organized a silent street protest against ISIS, hosting banners that "condemned the death of Dr. Rayan." He said ISIS fighters did nothing to stop the demonstration, preoccupied instead with fighting anti-ISIS rebels, and that the demonstrators "were happy after the clashes erupted."

Confirmation of the fighting in northern Syria was difficult because the area is particularly dangerous and inhospitable to outside journalists and foreign civilians, who have become increasingly vulnerable to kidnappings. Underlining the threat, Doctors Without Borders, the France-based medical charity that has operated in some areas of northern Syria, reported Friday that five of its staff members had been missing since Thursday.

The Syrian conflict began as a peaceful uprising against Mr. Assad in March 2011 and was widely supported by the United States and other European and Arab countries. It has since descended into a civil war with sectarian overtones that pit Mr. Assad's ruling minority Alawite group, an offshoot of Shiite Islam, against opposition forces drawn mostly from the country's Sunni majority. The Syrian Observatory has said more than 130,000 people have been killed.

ISIS, which has its roots in Iraq, was initially welcomed by many members of the insurgency and has become a significant force in the fighting against Mr. Assad, but its Qaeda affiliation has complicated efforts by the United States and others to help the opposition.