have complained to police and rivals Manchester City after flares were hurled into the away end at last Sunday's derby.
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.
They had asked for extra tickets but were disappointed when City handed over just 70 more.
Officials at the Etihad say their hands were tied and that the allocation was based on the police's categorisation of the match, which was deemed 'increased risk'.
Some at Old Trafford can see irony in the fact most of the resulting safety issues appear to have been caused by home supporters.
United are the only Premier League opponents to see their allocation cut, something which is not lost on staff at the club. United wrote to City to complain when a similar number of tickets was handed over for the derby in April of this year and are reluctant to let the issue lie.
Visiting fans were also locked in for a period of time at the end of the fixture, which Premier League leaders City won 3-1 to take them 12 points clear of their cross-town rivals.
Repeated messages have previously been sent out to supporters warning them not to bring flares to matches. Ahead of bonfire night weekend, the EFL asked clubs to make their fanbases aware of the dangers.
Last year, all 72 clubs signed up to a new Club Charter on Pyrotechnics and, subsequently, anyone found in breach of the Charter receives a minimum three-season ban.
Dogs trained to sniff out the devices are routinely deployed and part of the probe will inevitably focus on how City fans managed to smuggle them into the stadium undetected.
In August, one of Scotland's top police officers told the BBC it was 'only a matter of time' before a fan was killed by a pyrotechnic at a football match.
United confirmed that they had asked Greater Manchester Police and City to investigate, while a City spokesman confirmed that they were aware of the incidents and were assisting with the probe.
GMP failed to respond to a request for a comment.