(CNN) -- An al Qaeda splinter group withdrew from a border town in northern Syria on Friday, ceding control to rebel battalions for the first time in five months, a monitoring group said.
The group, part of an organization calling itself the Islamic State of Iraq and Syria, left the city of Azaz and surrounding villages where observers say a 16-year-old commander ruled by a radical interpretation of Islamic law.
Social media video purporting to depict the scene shows residents gathered in a public square chanting, "The Free Syria Army forever, with the people of Azaz" as newly returned fighters fire celebratory gunfire.
The retreat comes four days into a five-day ultimatum issued by the al Qaeda-linked al-Nusra Front demanding ISIS leave Syria.
"I swear by God, if you again refuse God's judgment, and do not stop your plague and pushing your ignorant ideology on the Muslim nation then you will be expelled, even from Iraq," Abu Mohammed al-Jolani, the chief of al-Nusra Front, said in an audio message posted online.
Earlier this month, even top al Qaeda leader Ayman al-Zawahiri weighed in on the situation, and the group's central leadership posted a notice online saying ISIS "is not a branch of the al Qaeda group."
The order to leave came shortly after a suicide bombing allegedly carried out by ISIS killed a mediator representing al-Zawahiri in northern Syria.
Last year, ISIS launched a successful offensive to wrest control of large swaths of northern Syria and push rebels out of key population centers like Azaz.
The aggression fueled a war within a war -- pitting the rogue group against al-Nusra Front and other rebel factions -- that has claimed the lives of at least 3,300 people since the start of this year, according to the Syrian Observatory for Human Rights.