AMMAN, Jordan -- Clashes erupted Friday between groups nominally united in their opposition to the Syrian government, with rebel fighters in the north attacking an Al Qaeda-linked group that has increasingly fought against them.
The combat comes as the opposition movements seeking to oust President Bashar Assad find themselves increasingly fractured.
The fighting began early Friday when the Al Qaeda-linked Islamic State of Iraq and Syria, or ISIS, tried to seize Atarib, a town near Aleppo held by other rebels, in order to capture the former base of the government's 46th Regiment and the courthouse, the rebels and activists said.
A newly formed grouping of Free Syrian Army militias called the Mujahedin Army prevented the ISIS from entering the town and captured almost two dozen of its fighters, including the local leader.
The rebels called on ISIS fighters to defect from the group.
For months, the ISIS has fought with the main rebel groups, seizing towns the opposition fighters controlled and creating a distraction for groups fighting to overthrow Assad. The rebels, many of them aligned with the Western-backed Free Syrian Army and with a newly formed Islamic Front, have at times tried to avoid open confrontation with the ISIS, preferring to focus on the battles with government forces.
Fighters with the Islamic Front, which has recently eclipsed the Free Syrian Army as the main rebel group, also fought alongside the Mujahedin Army in Atarib.
In a statement, the Islamic Front said: "ISIS comes to start a war against the fighting factions in the town of Atarib ... and we remind the soldiers of ISIS that the ones who liberated Atarib and the Aleppo countryside are those who you are fighting today."
From Atarib, the clashes spread into several neighborhoods in the city of Aleppo, where rebels expelled the ISIS fighters and arrested some of their ranks. Many were reported killed on both sides, though an accurate death toll was not immediately available.
In the city of Maarat Numan, in nearby Idlib province, rebel groups attacked the ISIS headquarters and arrested its fighters there.
"It's a war, not just battles," said Aleppo activist Nazeer Khatib.