Gaziantep, Turkey (CNN) -- Clashes continued to rage on the sixth day of what appears to be a coordinated offensive by Syrian rebels to push a hard-line al Qaeda militia out of northern Syria.
"They're shooting back and forth," said Selami Yilmaz, a resident of the Turkish border town of Karkamis. Gunfire echoed over the receiver as he spoke to CNN by phone Wednesday.
Yilmaz said he could see and hear fighting across the border in the nearby Syrian town of Jarablus, which has been controlled for months by the al Qaeda-linked militant group called the Islamic State of Iraq and Syria, or ISIS.
Jarablus is one of many towns across a swath of opposition-controlled northern Syria where rebels have attacked ISIS positions in recent days. The deadly power struggle marks a dramatic shift in the Syrian civil war, as many opposition fighters have turned from battling the forces of Syrian strongman Bashar al-Assad to attacking their own former comrades in arms.
Rebel spokesmen said that ISIS's violent enforcement of a plan to establish an Islamic state in Syria had alienated other opposition groups.
"They (ISIS) have become aggressive, and they have crossed the red line from trespassing to assaults to their extremist agenda," said Islam Aloush, a spokesman for a rebel umbrella group that calls itself the Islamic Front.
In an interview with CNN on Tuesday, Aloush said ISIS's radical interpretation of Sharia law, public executions and imprisonment of opponents were key factors that spurred the opposition to take up arms against the al Qaeda-linked group.
"ISIS cannot withstand the losses they are taking and the numbers now held as prisoners of war," Aloush said, saying that his organization far outnumbered ISIS. The Islamic Front boasts an estimated 40,000 fighters, making it probably the single largest rebel command, according to Aron Lund, editor of the Carnegie Endowment's Syria in Crisis website.
Fighters on both sides appear to have committed atrocities.
According to the London-based Syrian Observatory for Human Rights, rebels summarily executed at least 34 ISIS militants near the town of Jabal al-Zawiya this week.
Meanwhile, rebels posted videos and photos of the bodies of at least nine men who they said had been executed after being kidnapped by ISIS.
The graphic images, which were said to be filmed in the battlefield city of Aleppo, showed the men apparently shot through the head. Each victim's hands appeared to be bound from behind with plastic cuffs. CNN could not verify the authenticity of the photos and video.
The chaotic war within a war that erupted over the last week in northern Syria is further complicated by the fact that the anti-ISIS faction includes another militia tied to the al Qaeda terror network.
Militants from the rival al Nusra Front, which follows a hardline Islamist ideology inspired by al Qaeda, have joined in the battle against ISIS in recent days.
On Tuesday, Nusra's leader called for an end to the bloody infighting, which is believed to have left hundreds of people dead.
In a nine-minute audio message, Abu-Mohammad al-Jolani blamed "the incorrect policies" of ISIS for "aggravating the conflict" between armed opposition factions.
He proposed an immediate cease-fire and the creation of an Islamic council with representatives from all of the relevant groups.
CNN's Ivan Watson reported and wrote from Gaziantep. Raja Razek reported from Gaziantep. CNN's Salma Abdelaziz reported from Atlanta. CNN's Gul Tuysuz contributed to this report.