The paper deals with the obstacle avoidance problem for unmanned aerial vehicles (UAVs) operating in teleoperated mode. First, a feedback controller that we proposed recently for the stabilization of the UAV's linear velocity is recalled. Then, based on sensory measurements, a control strategy is proposed in order to modify the reference velocity on-line in the neighborhood of obstacles so as to avoid collisions. Both cases of telemetry and optical flow sensors are addressed. Stability properties of the proposed feedback controller are established based on a Lyapunov analysis. Simulations results are reported to illustrate the approach.