A Syrian activist group and rebels say there's infighting between rebel groups seeking the overthrow of President Bashar Assad.

Rami Abdurrahman of the Britain-based Syrian Observatory said Tuesday that some rebel groups were fighting against Syria's al-Qaida affiliate, the Nusra Front, in the northern province of Idlib, near the Turkish border.

A spokesman for some Western-backed groups also confirmed the infighting, as did a rebel group involved.

The infighting apparently began after Nusra Front fighters left a front threatened by government forces near the northern city of Aleppo.

If the fighting spreads, it may threaten rebel gains in Syria. Rebel forces are already badly weakened by government victories and infighting that began last winter against militants of an extremist group now called the "Islamic State."