The rebels are all armed and include religious fighters, according to the Syrian Observatory for Human Rights
At least 500 rebels on Wednesday reportedly crossed the Turkish border heading for the Syrian town of Azaz in northern Aleppo province, where opposition forces have suffered setbacks at the hands of Kurdish forces.
"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 religious fighters, all of them armed, he added.
This is the second time in days a large group of rebels has crossed the border, after almost 350 passed through the Atme border crossing on 14 February armed with heavy and light weapons.
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.
Alarmed by the Kurds' gains close to its border, Turkey, which supports the groups opposing Syrian President Bashar Al-Assad, continued shelling their positions in Syria for a fifth day.