This paper deals with robotic grasps exhibiting viscoelastic contact interactions with the manipulated object. Such interactions are frequent in advanced robotic applications since in many cases, i.e. when a great precision is needed, elasticity cannot be neglected. This is particularly true in medical applications, as in tele-surgery or in laparoscopy, where precision is obviously basic. The control of internal forces for visco-elastic grasps is explored in this work. This is an important issue in tele-operated robotics (tele-surgery and so forth). The presence of non-negligible compliance at the contacts, implies that the object dynamics cannot be neglected when attempting to control internal forces without affecting the object position. A geometric approach to derive a control law dynamically decoupling the internal force control action from the object dynamics is proposed.