Structural and optical properties of thin InGaAsN insertions in GaAs, grown by molecular beam epitaxy using an RF nitrogen plasma source, have been investigated. Nitrogen incorporation into InGaAs results in a remarkable broadening of the luminescence spectrum as compared with that of InGaAs layer with the same indium content. Correspondingly, a pronounced corrugation of the upper interface and the formation of well defined nanodomains are revealed in cross-sectional and plan-view transmission electron microscope (TEM) images, respectively. Raising the indium concentration in InGaAsN (N<1 %) to 35 % results in the formation of well defined separated three-dimensional (3D) islands. The size of the nanodomains proves that the InGaAsN insertions in GaAs should be regarded as quantum dot structures even in the case of relatively small indium concentrations (25 %) and layer thicknesses (7 nm), which are below the values required for a 2D-3D transition to occur in InGaAs/GaAs growth. Dislocation loops have been found in TEM images of the structures emitting at 1.3 µm. They are expected to be responsible for the degradation of the luminescence intensity of such structures in agreement with the case of long-wavelength InGaAs-GaAs quantum dots.