Manchester United have complained to police and rivals Manchester City after flares were hurled into the away end at last Sunday's derby.
Sportsmail understands that United expressed their concerns following three separate incidents at the Etihad Stadium, which saw City fans launch pyrotechnics into the area where the visiting supporters were seated.
No serious injuries were reported but police are investigating, and City say they will assist with their inquiries.
The situation has done little to ease tensions between the clubs over ticketing arrangements. United were again given a reduced allocation for the late-afternoon clash.