More than 160,000 civilians are trapped in the fighting, which also forced the evacuation of one of the few remaining hospitals in the area, run by the international medical organization Doctors Without Borders.
On Saturday, ISIS fighters staged two suicide bombings targeting "opposition forces" near Marea, ISIS said via its news agency, Aamaq.
Following the suicide bombings, ISIS (IS, ISIL and Daesh) militants entered Marea and fighting began inside the town, according to the London Based Syrian Observatory for Human Rights.
"The territorial gains around the rebel strongholds of Marea and Azaz, north of Aleppo city, are a blow to the Turkey and Saudi-backed rebels, who have been struggling to retain a foothold in the region while being squeezed by opponents from all sides."
They also demonstrated the Islamic State group's ability to stage major offensives and capture new areas, despite a string of recent losses in Syria and Iraq.
The ISIS offensive targeting Syrian opposition strongholds near the Turkish border began Thursday night.
US Backed "Moderate Rebels" Beheaded Couple of ISIS terrorists After Marea offensive
US Backed Moderate Rebels Beheaded Couple of ISIS Members After Marea offensive
On Friday, militants of the group captured six villages near Azaz, triggering intense fighting that trapped tens of thousands of civilians unable to flee to safety while Turkey's border remains closed. A few hundred fled west to the Kurdish-held enclave of Afrin.
US Backed Moderate Rebels Beheaded Couple of ISIS Members After Marea offensive
People are "terrified for their lives," the International Rescue Committee said in a statement. The group said it has received confirmed reports that at least four entire families, including women and children, were killed Friday on the outskirts of the town of Azaz.
"The advances brought the militants to within few kilometers of the rebel-held town of Azaz and cut off supplies to Marea further south. Marea has long been considered a bastion of moderate Syrian revolutionary forces fighting to topple President Bashar Assad."
The U.N. refugee agency said it was "deeply concerned" about the fighting affecting thousands of vulnerable civilians.
Azaz, which hosts tens of thousands of internally displaced people, lies north of Aleppo city, which has been divided between a rebel-held east and government-held west.
A route known as the Azaz corridor links rebel-held eastern Aleppo with Turkey. That has been a lifeline for the rebels since 2012, but a government offensive backed by Russian air power and regional militias earlier this year dislodged rebels from parts of Azaz and severed their corridor between the Turkish border and Aleppo.
The predominantly Kurdish Syrian Democratic Forces (SDF), who are fighting for their autonomy in the multilayered conflict, also gained ground against the rebels.
In recent months, Syrian rebel factions in Azaz - which include mainstream opposition fighters known as the Free Syrian Army along with some ultraconservative Islamic insurgent factions - have been squeezed between ISIL to the east and predominantly Kurdish forces to the west and south, while Turkey restricts the flow of goods and people through the border.
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