  We present Very Large Array 7 mm continuum observations of four Ultra-Compact
(UC)HII regions, observed previously at 1.3 cm, in order to investigate the
nature of the compact radio sources associated with these regions. We detected
a total of seven compact radio sources, four of them with thermal emission, and
two compact radio sources have clear non-thermal emission. The thermal emission
is consistent with the presence of an ionized envelope, either static (i.e.,
trapped in the gravitational radius of an associated massive star) or flowing
away (i.e., a photo-evaporative flow). On the other hand, the nature of the
non-thermal sources remains unclear and several possibilities are proposed. The
possibility that most of these compact radio sources are photo-evaporating
objects and the remaining ones more-evolved objects is consistent with previous
studies on UCHII regions.
