Abstract <p>The current–voltage curves (I–V curves) of superconducting niobium nitride (NbN) films are obtained at temperatures below the superconducting transition temperature (<i>T</i>_c) in a constant magnetic field. Regions of linear I–V curves with differential resistance independent of the magnetic field were observed in the temperature range from 0.7<i>T</i>_c to 0.95<i>T</i>_c for magnetic fields of up to <i>H</i> = 80 kOe. The resistive state of the films in this case is not described by the usual magnetic flux flow. The presence of such I–V curve regions is explained by an additional constant force acting on the vortex lattice. The origin of this force may be associated with collective pinning of the lattice on atomic-scale inhomogeneities, which are always present in niobium nitride, and with strong intervortex interaction. In this case, the movement of a weakly fixed part of the vortex lattice is possible in the environment of strongly pinned vortices near <i>T</i>_c in fields up to 5 kOe .</p>