Manchester City are in contact with Greater Manchester Police to identify a fan who appeared to make a racist gesture during Saturday's derby at the Etihad.
The fan seemingly mimicked a monkey while hurling epithets at Fred and Jesse Lingard. United were 2-0 up at the time.
"Manchester City FC are aware of a video circulating on social media which appears to show a supporter making racial gestures during the second half of the match against Manchester United this evening," read a statement from the club.
"Officials from the club are working with Greater Manchester Police in order to help them identify any individuals concerned and assist with their inquiries."
Kick It Out, the anti-racism watchdog that promotes equality in the English game, said it was "inundated with reports of alleged racist abuse from a number of individuals."
Several City players, including Fernandinho, motioned to the crowd to settle down. Referee Anthony Taylor resumed play after clearing the field of debris.
United manager Ole Gunnar Solskjaer called for the fan to be banned indefinitely.
"I've seen it on video, it was Jesse and Fred, and the fella must be ashamed of himself," Solskjaer told BBC Match of the Day. "It's unacceptable and I hope he will not be watching any football anymore."
Marcus Rashford, who was subjected to racism while playing for England in Bulgaria in October, noted the alarming regularity of these incidents.
"We seem to be speaking about it an awful lot over (the) last six to eight months," he said. "Even speaking about it now is not nice. The necessary departments need to do the right things to stop it in the game. It is a big negative in the sport and the country."