BEIRUT: Al-Qaida militants fought heavy street battles against Kurdish gunmen in northern Syria on Friday but reached a cease-fire with mainstream Western-backed rebels after fierce fighting near the border with Turkey.
The infighting, which ended with al-Qaida's takeover of a border town, was some of the worst in recent months between forces seeking to bring down President Bashar Assad and threatened to further fragment an opposition movement outgunned by the regime.
Syria's main Western-backed opposition group, the Free Syrian Army, condemned the Islamic State in Iraq and the Levant, the al-Qaida offshoot that overran the town of Azaz. It said the jihadis' push to establish an Islamic state undermines the rebels' struggle for a free Syria.
In a strongly worded statement, it accused ISIL of going against the principles of the Syrian revolution.
"ISIL no longer fights the Assad regime. Rather, it is strengthening its positions in liberated areas at the expense of the safety of civilians," the statement said. "ISIS is inflicting on the people the same suppression of the Baath party and the Assad regime."
Al-Qaida-linked fighters in Syria have been some of the most effective forces on the battlefield, fighting alongside the Free Syrian Army against government forces.
But the two factions have turned their guns on each other and turf wars and retaliatory killings have evolved into ferocious battles that have effectively become a war within a war in northern and eastern Syria, leaving hundreds dead on both sides.
Late Thursday, fighters from the ISIL and the mainstream rebel FSA agreed on an immediate cease-fire in Azaz near the border with Turkey, activists and opposition groups said.
The two sides also agreed to free fighters captured by each side, according to the British-based Syrian Observatory for Human Rights.
The deal calls for setting up a checkpoint between the two sides. They also agreed to take disagreements before an Islamic council that would soon be established to resolve disputes.
The fighting in Azaz and the prospect of al-Qaida militants so close to the frontier prompted Turkey to close a nearby border crossing.
Veteran opposition figure Kamal Labwani said the international community's disregard for Syrian lives has strengthened extremists in Syria, adding that the ISIL has become a force that the FSA is unable to deal with.
ISIL "invaded Azaz in one hour. Nobody can confront such extremists. They know how to work, they know how to plan," he said.
Labwani said the FSA had no choice but to agree to a truce because it cannot afford to open another front.
The extremists' presence "has spread like a disease which cannot be stopped," he said.
But as the fighting in Azaz died down, ISIL fighters fought heavy streets battles against Kurdish gunmen in the northern province of Raqqa, the Observatory said. Such battles between the two groups have been common in the past months.
ISIL members in Raqqa publicly shot to death an army officer they had captured earlier because he belongs to Assad's minority Alawite sect, an offshoot of Shiite Islam, Abdul-Rahman said.
Abdullah Hassan, official spokesman for the local council in Raqqa, said via Skype that "all armed battalions and fighters, as well as civilians, are opposed to ISIL."
"These people do not have the same goals as us. We didn't liberate Azaz for them to come and occupy it again only this time with the rule of Islam," he said referring to the town that was among the first areas in northern Syria to fall into the hands of rebels.
Also Friday, state-run news agency SANA said Syrian Deputy Prime Minister Qadri Jamil was misquoted in an interview with the Guardian in which he said that neither side in Syria was strong enough to win the conflict and that the government will call for a cease-fire at a planned peace conference in Geneva.