In this paper we study the problem of optimal placement for a team of mobile robots with surveillance tasks in an environment with unknown obstacles. In particular, we introduce a new approach which is based on the combination of the Voronoi partition and the potential field method. This allows obtaining distributed control which overcomes the drawbacks of these two methods, as stand-alone methods. In particular, the proposed approach is able to deal with any kind of environment. Extensive simulations are presented to show the performance of our algorithm.