Passivity-based control of underactuated mechanical systems with nonlinear friction effects is analyzed. The port-controlled-Hamiltonian equations of the mechanical system are extended by incorporating a novel dynamic friction model, whose internal states are included as generalized coordinates. The extended Hamiltonian model preserves passivity properties of the LuGre model for the velocity-force pair, with no extra conditions on the friction model parameters. Simulation results are presented for an underactuated double pendulum.