Manchester City say they are investigating claims of alleged racist abuse by fans during their loss to Manchester United at the Etihad Stadium.
The club released a statement in the aftermath of their derby loss to Manchester United, saying the club was "aware of a video circulating on social media" which appeared to show a supporter engaging in racist behaviour.
The club say they will punish anyone found guilty of racist abuse with a lifetime ban.
There was also an incident in the second half when it appeared Manchester United player Fred was struck by an object thrown from the stands.
The statement said:
"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.
Officials from the Club are working with Greater Manchester Police in order to help them identify any individuals concerned and assist with their enquiries.
"The Club are also working with GMP regarding an instance of objects being thrown onto the field of play.
"The Club operates a zero tolerance policy regarding discrimination of any kind, and anyone found guilty of racial abuse will be banned from the club for life."
City were beaten 2-1 by United in the latest installment of the Manchester derby, with the reigning champions now 14 points adrift of Premier League leaders Liverpool.
First-half goals from Marcus Rashford - who scored from the penalty spot after the intervention of VAR - and Anthony Martial gave United a two-goal cushion.
City pulled one back through a Nicolas Otamendi header in the 85th minute, but United held on.
Ole Gunnar Solskjaer's side now move up to fifth and within five points of fourth-placed Chelsea.
City remain in third place and could find themselves six points off second if Leicester City beat Aston Villa tomorrow.