The Ecuadorian, who is 20, may be banned for two years due to a doping charge.
Zambrano signed for Hull on loan from LDU Quito in the summer, despite having been charged.
Zambrano is understood to have tested positive for an anti-allergy medication that he took without telling his club’s doctors.
A spokesperson for the Championship club stated this week: “Hull City can confirm that, with immediate effect, Conmebol, the South American Football Confederation, has suspended Oscar Zambrano for breaching the anti-doping rules set out in Articles 6 and 7 of the Conmebol Anti-Doping Regulations.
“Zambrano and his defence will use their right to appeal this decision to the Court of Arbitration for Sport.
“The club is not in a position to provide further information at this time.”