  This paper reports 43 GHz imaging of the high-mass protostars W33A, AFGL 2591
and NGC 7538 IRS9 at 0.04'' and 0.6'' resolution. In each case, weak (~mJy),
compact (~100 AU) emission is detected, which has an elongated shape (axis
ratio ~3). For AFGL 2591 and NGC 7538 IRS9, the emission is single-peaked,
while for the highest luminosity source, W33A, a `mini-cluster' of three
sources is detected. The derived sizes, flux densities, and broad-band radio
spectra of the sources support recent models where the initial expansion of HII
regions around very young O-type stars is prevented by stellar gravity. In
these models, accretion flows onto high-mass stars originate in large-scale
molecular envelopes and become ionized close to the star. These models
reproduce our observations of ionized gas as well as the structure of the
molecular envelopes of these sources on 10^3--10^4 AU scales derived previously
from single-dish submillimeter continuum and line mapping. For AFGL 2591, the
43 GHz flux density is also consistent with dust emission from a disk seen in
near-infrared `speckle' images. However, the alignment of the 43 GHz emission
with the large-scale molecular outflow argues against an origin in a disk for
AFGL 2591 and NGC 7538 IRS9. In contrast, the outflow from W33A does not appear
to be collimated. Together with previously presented case studies of W3 IRS5
and AFGL 2136, our results indicate that the formation of stars and stellar
clusters with luminosities up to ~10^5 L0 proceeds through accretion and
produces collimated outflows as in the solar-type case, with the `additional
feature' that the accretion flow becomes ionized close to the star. Above ~10^5
L0, clusters of HII regions appear, and outflows are no longer collimated,
possibly as the result of mergers of protostars or pre-stellar cores.
