WARNING: Graphic language.

Police sources say an ­investigation is under way after a video surfaced Thursday of what appears to be a plainclothes cop punching a youngster multiple times during an arrest on the Lower East Side.

The incident unfolded Monday shortly before 4 p.m. when several uniformed cops arrested two youngsters who they believed had beaten another boy with a cane.

While apprehending one of the boys — who onlookers say looked to be about 12 years old — the plainclothes cop rushes toward him and is seen assaulting the young suspect while assisting the uniformed officers.

Sources say the NYPD ­Internal Affairs Bureau is probing the incident.