High-resolution imaging of ultracompact HIIregions***

We present the first near-infrared images of the two massive southern star-forming regions G11.11-0.40 and G341.21-0.21, obtained with adaptive optics at sub-arcsecond resolution. Together with conventional narrow-band near-infrared images and polarization maps, mid-infrared data, millimetre and radio data, we were able to determine the morphological structure and to constrain the stellar content of the regions. In both cases the molecular cloud cores have masses of about 500 ${{M}_{\odot}}$and contain clusters of OB stars. This supports the view that massive stars do not form in isolation. In the case of G11.11-0.40 we could identify the central O star responsible for the ionization of the ultracompact Hiiregion. Our results for G341.21-0.21 indicate that this source is a precursor of an ultracompact Hiiregion.