The are also being investigated by both the Premier League and the Football Association in England and by Uefa, the governing body of European football.
The FA said in February it was looking into allegations that City paid £200,000 ($256,000) to winger Jadon Sancho's agent when the player was 14 and still at Watford.
FA rules state that young players cannot be represented by an agent until the year they turn 16.
Sancho later left City for Borussia Dortmund. He is now an England international.
Uefa is focusing on another area and investigating whether City has breached Financial Fair Play rules.
City's English rivals Chelsea are currently serving a one-year transfer ban after also being found guilty by the body in February of breaching regulations relating to the recruitment of minors.
Barcelona, Real Madrid and Atletico Madrid have all served recent transfer bans for breaking Fifa rules on the issue.