An increasingly popular approach to support military forces deployed in urban environments consists in using autonomous robots to carry on critical tasks such as mapping and surveillance. In order to cope with the complex obstacles and structures found in this operational context, robots should be able to perceive and analyze their world in 3D. The method presented in this paper uses a 3D volumetric sensor to efficiency map and explore urban environments with an autonomous robotic platform. A key feature of our work is that the 3D model of the environment is preserved all along the process using a multiresolution octree. This way, every module can access the information it contains to achieve its tasks. Simulation and real word tests were performed to validate the performance of the integrated system and are presented at the end of the paper.