"At least 500 rebels have crossed the Bab al-Salam border crossing on their way to the town of Azaz yesterday, 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.
Opposition forces have lost ground to government troops in northern Aleppo province since they began a major offensive in the former rebel bastion backed by Russian air strikes.
Pressing their advantage, Kurdish forces have been taking ground in the fragmented region, most notably the city of Tal Rifaat, as they seek to carve out their own autonomous state.
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.