The armed group were predominantly drawn from the "Sham legion", otherwise known as the Homs Legion, who are described as a moderate Islamist group, media activist Majed Abdul Nour told The New Arab.
"At least 500 rebels have crossed the Bab al-Salam border crossing on their way to the town of Azaz, from which they want to help the insurgents in the face of gains made by Kurdish forces in the north of the province," the head of the Syrian Observatory for Human Rights, Rami Abdel, told AFP.
They include rebels as well as Islamist fighters, all of them armed, he added.
|Azaz is very close to the Turkish border|
Opposition forces have lost ground to government troops in northern Aleppo province since Damascus began a major offensive in the former rebel bastion, supported by Russian air strikes and Iranian militias.
YPG Kurdish forces, also reportedly supported by Russia, have been taking ground in the fragmented region, most notably the city of Tal Rifaat, as they seek to carve out their own autonomous region with ambitions for statehood.
Reports say that the Kurdish fighting force has already attempted to rename Tel Rifaat as "Arfat", the Kurdish name for the Arab-majority area.
The rebels now hold only the town of Azaz, not far from the Turkish border, and to the south Marea, which is currently almost encircled by Kurdish forces to the west and Islamic State fighters to the east.
Alarmed by the Kurds' gains close to its border, Turkey, which supports other groups opposing Syrian President Bashar al-Assad, continued shelling their positions in Syria for a fifth day. Ankara also targeted Kurdish positions in Iraq on Thursday, as a car bomb in the Turkish capital hours earlier was blamed on Kurdish militants.
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Meanwhile, Ahfad Saladin, a Kurdish faction allied with the Free Syrian Army in the area has denounced the YPG's actions.
"We as Kurdish rebels, as part of the FSA and the Syrian revolution and in defence of the Syrian people in general and the Syrian Kurdish people in particular, we will fight the... alliance until the last drop of our blood," the faction said in a statement.
Regime airstrikes continued to target the area of Thursday, with local activists reporting a number of barrel bombs targeting the town of Hayan, north of Aleppo.