As the cease-fire took hold, al-Qaida militants fought heavy street battles against Kurdish gunmen in northern Syria.
The infighting was some of the worst in recent months between forces seeking to bring down President Bashar Assad, and it threatened to further fragment an opposition movement outgunned by the regime.
The Syrian National Coalition, the main opposition group, condemned ISIL in a strongly worded statement, saying the jihadis' push to establish an Islamic state goes 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. "ISIL 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 rebels' 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 ISIL and the Free Syrian Army agreed on an immediate cease-fire in Azaz, 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 disputes before an Islamic council that would soon be established.
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 that cannot be stopped," he said.
But as the fighting in Azaz died down, ISIL fighters fought against Kurdish gunmen in in heavy streets battles in the northern province of Raqqa, the Observatory said. Such battles between the two groups have been common in the past months.
Kurds are the largest ethnic minority, making up more than 10 percent of the country's 23 million people, and were long oppressed by Assad's regime. When the revolt began in March 2011, some Kurds joined the peaceful protests against Assad's rule. But as the revolt shifted into an armed rebellion, many remained on the fence, suspicious of an opposition that was becoming increasingly dominated by Muslim extremists seeking to impose a strict interpretation of Islam.
Syria's Kurds also find themselves enjoying near autonomy in the northeast after overstretched regime forces pulled back, ceding de facto control to armed Kurdish fighters.
But clashes have erupted in the Kurdish-controlled areas with increasing frequency in recent months, pitting Kurdish militias against rebels from two al-Qaida-linked factions - Jabhat al-Nusra and the Islamic State in Iraq and the Levant. Following the killing of a prominent Kurdish leader late last month, a powerful Kurdish militia said it was mobilizing to expel Islamic extremists.
ISIL members in Raqqa also 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, said Rami Abdul-Rahman, who heads the Observatory.
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.