This paper addresses the forbidden state problem of Petri nets with liveness requirement and uncontrollable transitions. The proposed approach synthesizes in two steps an optimal Petri net-based controller, whenever such a controller exists. First, the behavior of the closed loop system is computed with a state space search. Then, control places that realize the computed behavior when added to the original model, are synthesized. Necessary and sufficient condition of the existence of such places is obtained. A manufacturing application of the method shows its efficiency. || Nous proposons, dans cet article, une méthode de synthèse de superviseurs non bloquants basés sur les réseaux de Petri, pour résoudre le problème d'états interdits en présence d'événements non contrôlables. L'approche proposée dans cet article combine l'a