The group of 500 opposition fighters was in the border town of Azaz, after arriving from Turkey on Wednesday through the nearby Bab al-Salama crossing, the Britain-based Syrian Observatory for Human Rights said.
The fighters are expected to head to frontlines nearby with the Syrian Democratic Forces (SDF), which has in recent days seized several former rebel bastions in Aleppo province.
The alliance is led by the Kurdish People's Protection Units (YPG), and its advances have alarmed Turkey, which considers the YPG a branch of the Kurdistan Workers' Party that has waged a decades-long insurgency against Ankara.
The Observatory said the Syrian rebels were a mixture of Islamists and other fighters, with most from the Faylaq al-Sham group.
It said they had arrived with weapons, though it could not provide details.
"They came from (neighbouring) Idlib province and western Aleppo, entered Turkey through (Idlib's) Atme, and reentered Syria through Bab al-Salama," Observatory chief Rami Abdel Rahman said.
The route allowed the rebels to avoid crossing Kurdish or regime-held territory to reach northern Aleppo, where the SDF and pro-government forces have recently advanced.
Syria expert Thomas Pierret described Faylaq al-Sham as "the official military branch of the Muslim Brotherhood... a faction that is close to Turkey."
He said their arrival could help reinforce Azaz, but might be insufficient to roll back the SDF's gains.
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"These reinforcements could contribute to stopping the fall of Azaz, but considering the Russian aerial support the YPG benefits from, I doubt that they will be pushed from most of the positions they captured in recent days," he said.
The SDF denies coordinating with Russian forces who began an air campaign in support of Syria's President Bashar al-Assad in late September.
But Russian warplanes have carried out strikes benefitting the alliance as it has advanced, and its successes have come as regime troops backed by Russia's air power have pressed their own major military operation further south in Aleppo province.
- Hasakeh, Latakia gains -
The Kurds have long sought to unite Kurdish-majority areas in north and northeast Syria, and their latest advances could help link the areas under their control.
Turkey is fiercely opposed to that goal, and has been shelling SDF positions inside Syria for days in a bid to halt the group's advances.
In its latest gains, the SDF, which launched an offensive Tuesday in the northeastern province of Hasakeh, has seized control of seven villages from jihadist fighters, according to the Observatory.
Elsewhere in the country, Syrian state media said regime forces had taken the village of Kinsaba in northern Latakia province, the last remaining rebel stronghold in the region.
The Observatory said fighting was ongoing but confirmed that large parts of the village had been captured by regime forces.
According to a Syrian military source on the ground, regime forces backed by Russian air power now control more than 70 percent of the north of Latakia province.
The regime advances in the Kinsaba area bring them closer to Jisr al-Shughur in Idlib province, which has been a key goal for government forces since it fell to rebels last April.