  We observed the W28 field (for ~40 h) at Very High Energy (VHE) gamma-ray
energies (E>0.1 TeV) with the H.E.S.S. Cherenkov telescopes. A reanalysis of
EGRET E>100 MeV data was also undertaken. Results from the NANTEN 4m telescope
Galactic plane survey and other CO observations have been used to study
molecular clouds. We have discovered VHE gamma-ray emission (HESSJ1801-233)
coincident with the northeastern boundary of W28, and a complex of sources
(HESSJ1800-240A, B and C) ~0.5 deg south of W28, in the Galactic disc. The VHE
differential photon spectra are well fit by pure power laws with indices
Gamma~2.3 to 2.7. The NANTEN ^{12}CO(J=1-0) data reveal molecular clouds
positionally associating with the VHE emission, spanning a ~15 km s^{-1} range
in local standard of rest velocity. The VHE/molecular cloud association could
indicate a hadronic origin for HESSJ1801-233 and HESSJ1800-240, and several
cloud components in projection may contribute to the VHE emission. The clouds
have components covering a broad velocity range encompassing the distance
estimates for W28 (~2 kpc), and extending up to ~4 kpc. Assuming a hadronic
origin, and distances of 2 and 4 kpc for cloud components, the required cosmic
ray density enhancement factors (with respect to the solar value) are in the
range ~10 to ~30. If situated at 2 kpc distance, such cosmic ray densities may
be supplied by a SNR like W28. Additionally and/or alternatively, particle
acceleration may come from several catalogued SNRs and SNR candidates, the
energetic ultra compact HII region W28A2, and the HII regions M8 and M20 along
with their associated open clusters. Further sub-mm observations would be
recommended to probe in detail the dynamics of the molecular clouds at
velocites >10 km s^{-1}, and their possible connection to W28.
